Echo canceller and method for controlling echo cancellation

Claims

What is claimed is:

1. An echo canceller which cancels echo generated at 2-wire/4-wire conversion portion by impedance mismatching comprising:

(a) an echo cancelling means for cancelling echo signal;

(b) a pattern inserting means for extracting a predetermined portion of a receiving side pulse code modulation (PCM) input signal and inserting a predetermined bit-pattern sequence into a receiving side PCM output signal in place of the extracted portion; and

(c) a pattern detecting means for detecting a predetermined bit pattern by monitoring a transmitting side input PCM signal, and disabling the echo cancelling function of the echo cancelling means when the predetermined bit-pattern is detected.

2. An echo canceller which cancels echo generated at 2-wire/4-wire conversion portion by impedance mismatching comprising:

(a) an echo cancelling means for cancelling echo signal;

(b) a first channel associated signal detecting means for determining whether a voice channel at the receiving side is in an idle state;

(c) a second channel associated signal detecting means for determining whether a voice channel at the transmitting side is in an idle state;

(d) a pattern detecting means for detecting a predetermined bit-pattern in a transmitting side input pulse code modulation (PCM) signal;

(e) a pattern inserting means for inserting a predetermined bit-pattern sequence into a receiving side PCM signal; and

(f) an operation controlling means for disabling echo cancelling function of the echo canceling means after detection of the predetermined bit-pattern in the transmitting side input PCM signal until the voice channel is in an idle state as detected by both the first and second channel associated signal detecting means, and for enabling the pattern inserting means for a predetermined duration when the voice channel status changes from the idle state to a busy state as detected by both the first and second channel associated signal detecting means.

3. An echo canceller which cancels echo generated at 2-wire/4-wire conversion portion by impedance mismatching comprising:

(a) an echo cancelling means for cancelling echo signal;

(b) a first common channel signal detecting means for determining whether a voice channel at the receiving side is in an idle state;

(c) a second common channel signal detecting means for determining whether a voice channel at the transmitting side is in an idle state;

(d) a pattern detecting means for detecting a predetermined bit-pattern in a transmitting side input pulse code modulation (PCM) signal;

(e) a pattern inserting means for inserting a predetermined bit-pattern sequence into a receiving side PCM signal; and

(f) an operation controlling means for disabling echo cancelling function of the echo canceling means after detection of the predetermined bit-pattern in the transmitting side input PCM signal until the voice channel is in an idle state as detected by both the first and second common channel signal detecting means, and for enabling the pattern inserting means for a predetermined duration when the voice channel status changes from the idle state to a busy state as detected by both the first and second common channel signal detecting means.

4. An echo canceller which cancels echo generated at 2-wire/4-wire conversion portion by impedance mismatching, where a channel associated signaling signal or a common channel signaling signal is not included in receiving side or transmitting side input signal bit streams comprising:

(a) an echo cancelling means for cancelling echo signal;

(b) a signaling status input to obtain a channel status indicating whether a relevant voice channel is in an idle state or in a busy state.

(c) a pattern inserting means for inserting a predetermined bit-pattern into a receiving voice channel pulse code modulation (PCM) signal when the signaling status input indicates that the relevant voice channel status changed from an idle state to a busy state;

(d) a pattern detecting means for detecting a predetermined bit pattern by monitoring a transmitting side input PCM signal; and

(e) an operation controlling means for disabling echo cancelling function of the echo cancelling means after detection of the predetermined bit-pattern in the transmitting side input PCM signal until the voice channel status input changes from a busy state to an idle state, and for enabling the predetermined bit-pattern inserting means for a predetermined duration after the voice channel status changes from an idle state to a busy state.

5. An echo canceller which cancels echo generated at 2-wire/4-wire conversion portion by impedance mismatching comprising:

(a) an echo cancelling means for cancelling echo signal;

(b) a first pattern detecting means for detecting a predetermined bit pattern in a transmitting side input pulse code modulation (PCM) signal, and disabling the echo cancelling function of the echo cancelling means when the predetermined bit-pattern is detected;

(c) a second pattern detecting means for detecting a predetermined bit pattern by monitoring receiving side input PCM signal bit stream; and

(d) a pattern inserting means for inserting a predetermined a bit pattern into a receiving side voice channel PCM signal when the second pattern detecting means does not detect the predetermined bit-pattern.

6. An echo canceller which cancels echo generated at 2-wire/4-wire conversion portion by impedance mismatching comprising:

(a) an echo cancelling means for cancelling echo signal, which is disabled when a predetermined bit-pattern is detected in a transmitting side input pulse code modulation (PCM) signal, or while a data call is applied to a voice channel;

(b) a pattern inserting means for inserting a predetermined bit pattern in a receiving side PCM signal while the data call is not applied to the voice channel;

(c) a pattern detecting means for detecting the predetermined bit-pattern in the transmitting side input PCM signal;

(d) a tone detecting means for detecting a modem answering tone by monitoring the transmitting side PCM signal and a receiving side input PCM signal;

(e) a power detecting means for detecting an end of a data call by monitoring the absence of signal power for more than predetermined period in both the transmitting and receiving side input PCM signals; and

(f) a register for applying the data call to the voice channel after the modem answering tone signal is detected until the power detecting means detects the end of the data call.

7. An echo canceller which cancels echo generated at 2-wire/4-wire conversion portion by impedance mismatching, where a voice channel is not coded by low bit rate coding comprising:

(a) an echo cancelling means for cancelling echo signal;

(b) a first channel associated signal detecting means for determining whether the voice channel at the receiving side is in an idle state;

(c) a second channel associated signal detecting means for determining whether the voice channel at the transmitting side is in an idle state;

(d) a pattern inserting means for inserting a predetermined bit-pattern in the receiving side pulse code modulation (PCM) signal;

(e) a pattern detecting means for detecting a predetermined bit-pattern in a transmitting side input PCM signal;

(f) a tone detecting means for detecting a modem answering tone by monitoring both the transmitting side input PCM signal and a receiving side input PCM signals; and

(g) an operation controlling means for disabling an echo cancelling function of the echo cancelling means after detection of the predetermined bit-pattern in the transmitting side input PCM signal at the pattern detecting means or after detection of the modem answering tone, until the voice channel status changes from a busy state to an idle state as detected by both the first and the second channel associated signal detecting means, and for enabling the pattern inserting means for a predetermined duration when the voice channel status changes from an idle state to a busy state as detected by both the first and second channel associated signal detecting means, and disabling the pattern inserting means when the modem answering tone is detected.

8. An echo canceller which cancels echo generated at 2-wire/4-wire conversion portion by impedance mismatching comprising:

(a) an echo cancelling means for cancelling echo signal;

(b) a first common channel signal detecting means for determining whether a voice channel at the receiving side is in an idle state;

(c) a second common channel signal detecting means for determining whether the voice channel at the transmitting side is in an idle state;

(d) a pattern inserting means for inserting a predetermined bit-pattern in a receiving side pulse code modulation (PCM) signal;

(e) a pattern detecting means for detecting a predetermined bit-pattern in a transmitting side input PCM signal;

(f) a tone detecting means for detecting a modem answering tone by monitoring both the transmitting side input PCM signal and a receiving side input PCM Signal; and

(g) an operation controlling means for disabling an echo cancelling function of the echo cancelling means after detection of the predetermined bit-pattern in the transmitting side input PCM signal at the pattern detecting means or after detection of the modem answering tone, until the voice channel status changes from a busy state to an idle state as detected by both the first and the second channel associated signal detecting means, and for enabling the pattern inserting means for a predetermined duration when the voice channel status changes from an idle state to a busy state as detected by both the first and second channel associated signal detecting means, and disabling the pattern inserting means when the modem answering tone is detected.

9. An echo canceller which cancels echo generated at 2-wire/4-wire conversion portion by impedance mismatching comprising:

(a) an echo canceller circuit for cancelling echo signal;

(b) a signaling status input to obtain a channel status indicating whether a relevant voice channel is in an idle state or in a busy state;

(c) a pattern inserting means for inserting a predetermined bit-pattern into a receiving voice channel pulse code modulation (PCM) signal for a predetermined duration when the signaling status input indicates that the relevant voice channel status changed from an idle state to a busy state;

(d) a pattern detecting means for detecting a predetermined bit pattern by monitoring a transmitting side input PCM signal;

(e) a tone detecting means for detecting a modem answering tone by monitoring both transmitting side and receiving side input PCM signals; and

(f) an operation controlling means for disabling echo cancelling function of the echo cancelling means after detection of the predetermined bit-pattern in the transmitting side input PCM signal or after detection of the modem answering tone until the voice channel status input changes from a busy state to an idle state, for enabling the predetermined bit-pattern inserting means for a predetermined duration after the voice channel status changes from an idle state to a busy state, and for disabling the pattern inserting means when the modem answering tone is detected.

10. An echo canceller which cancels echo generated at 2-wire/4-wire conversion portion by impedance mismatching comprising:

(a) an echo cancelling means for cancelling echo signal, which is disabled when a predetermined bit-pattern is detected in a transmitting side input pulse code modulation (PCM) signal, or while a data call is applied to a voice channel;

(b) a first pattern detecting means for detecting the predetermined bit-pattern in the transmitting side input PCM signal bit stream, and disabling the echo canceling function of the echo canceling means while the predetermined bit-pattern is detected;

(c) a second pattern detecting means for detecting a predetermined bit-pattern in the receiving side input PCM signal;

(d) a pattern inserting means for inserting a predetermined bit pattern in a receiving side PCM signal while the predetermined bit-pattern is not detected at the receiving side input PCM signal or while the data call is applied to the voice channel;

(e) a tone detecting means for detecting a modem answering tone by monitoring the transmitting side input PCM signal and a receiving side input PCM signal;

(f) a power detecting means for detecting an end of a data call by monitoring the absence of signal power for more than predetermined period in both the transmitting and receiving side input PCM signals; and

(g) a register for applying the data call to the voice channel after the modem answering tone signal is detected until the power detecting means detects the end of the data call.

11. An echo canceller which cancels echo generated at 2-wire/4-wire conversion portion by impedance mismatching comprising:

(a) an echo cancelling means for cancelling echo signal, which is disabled when a predetermined bit-pattern is detected in a transmitting side input pulse code modulation (PCM) signal, or while a data call is applied to a voice channel;

(b) a first pattern detecting means for detecting the predetermined bit-pattern in the transmitting side input PCM signal;

(c) a second pattern detecting means for detecting a predetermined bit pattern in the receiving side input PCM signal;

(d) a pattern inserting means for inserting a predetermined bit pattern in a receiving side PCM signal while the predetermined bit-pattern is not detected at the receiving side input PCM signal or while the data call is applied to the voice channel;

(e) a modem signal detection input which receives detection information of a modem answering tone from an external tone detecting apparatus;

(f) a power detecting means for detecting an end of a data call by monitoring the absence of signal power for more than predetermined period in both the transmitting and receiving side input PCM signals; and

(g) a register for applying the data call for a duration after receipt of detection information by the modem signal detection input.

12. An echo canceller which cancels echo generated at 2-wire/4-wire conversion portion by impedance mismatching comprising:

(a) an echo cancelling means for cancelling echo signal;

(b) a first common channel signal detecting means for determining whether the voice channel at the receiving side is in an idle state;

(c) a second common channel signal detecting means for determining whether the voice channel at the transmitting side is in an idle state;

(d) a pattern inserting means for inserting a predetermined bit-pattern in the receiving side pulse code modulations (PCM) signal;

(e) a pattern detecting means for detecting a predetermined bit-pattern in a transmitting side input PCM signal;

(f) a tone detecting means for detecting a modem answering tone by monitoring both the transmitting side input PCM signal and a receiving side input PCM signals; and

(g) an operation controlling means for disabling an echo cancelling function of the echo cancelling means after detection of the predetermined bit-pattern in the transmitting side input PCM signal at the pattern detecting means or after detection of the modem answering tone until the voice channel status changes from a busy state to an idle state as detected by both the first and the second common channel signal detecting means, for enabling the pattern inserting means for a predetermined duration when the voice channel status changes from an idle state to busy state as detected by both the first and second common channel signal detecting means, and for disabling the pattern inserting means when the modem answering tone is detected.

13. An echo canceller according to any one of claims 1-12, wherein

said pattern inserting means is arranged between exchanges of a channel associated signaling system and periodically extracts second bits from least significant bits of receiving side PCM signal code-words and inserts the predetermined bit-pattern into the receiving side PCM signal code-words in place of the extracted second bits.

14. An echo canceller according to any one of claims 1-12, wherein

said pattern inserting means is arranged between exchanges of a channel associated signaling system and extracts least significant bits of receiving side PCM signal code-words with a period which has a prime number relation to a period of a PCM robbed-bit signaling system and inserts the predetermined bit-pattern into the receiving side PCM signal code-words in place of the extracted bits.

15. A method for controlling echo cancellation in an echo canceller which cancels echo generated at 2-wire/4-wire conversion portion by impedance mismatching, said method comprising the steps of:

(1) inserting a predetermined bit-pattern sequence in a receiving side pulse code modulation (PCM) signal; and

(2) disabling said echo cancellation when the predetermined bit-pattern is detected in a transmitting side input PCM signal.

16. The method of claim 15, further comprising the steps of:

(3) detecting whether a voice channel is in an idle state;

wherein said pattern is inserted upon detecting a change from an idle state to a busy state; and

wherein said echo cancellation is disabled until a change from a busy state to an idle state is detected.

17. The method of claim 15, wherein said insertion step includes periodically extracting least significant bits from the receiving side input PCM signal code-words and inserting the predetermined bit-pattern sequence.

18. The method of claim 15, wherein said inserting step includes periodically extracting second bits from least significant bits of the receiving side input PCM signal and inserting the predetermined bit-pattern sequence.

19. The method of claim 15, wherein said inserting step includes extracting least significant bits of the receiving side input PCM signal code-words with a period which has a prime number relation with a period of a PCM robbed bit signaling system.

20. The method of claim 15, further comprising the steps of:

(3) determining when a disable tone is present;

(4) stopping said inserting step when said disable tone is present; and wherein said echo cancellation is disabled when said disable tone is present.

21. The method of claim 20, wherein said disable tone is a modem answering tone.

22. The method of claim 20, wherein said disable tone is a 2100 Hz sinusoidal signal.

23. The method of claim 20, wherein said disable tone is a 2100 Hz sinusoidal signal with periodical phase reversal.

24. The method of claim 15, further comprising the steps of:

(3) detecting the predetermined pattern by monitoring the receiving side input PCM signal;

(4) stopping said inserting step in response to said predetermined pattern being detected in the receiving side input PCM signal.

25. The method of claim 24, further comprising the steps of:

(5) determining when a disable tone is present;

(6) disabling said inserting step when said disable tone is present; and wherein said echo cancellation is disabled when said disable tone is present.

26. An echo canceller which cancels echo generated at 2-wire/4-wire conversion portion by impedance mismatching comprising:

(1) an echo cancelling means for cancelling echo signal;

(2) a predetermined bit-pattern sequence insertion means for inserting a pattern in a receiving side pulse code modulation (PCM) signal; and

(3) operation control means for disabling said echo cancelling means when the pattern is detected in transmitting side input PCM signal.

27. The echo canceller of claim 26, further comprising:

(4) pattern detection means for detecting said predetermined bit-pattern in transmitting side input PCM signal.

28. The echo canceller of claim 26, wherein said pattern insertion means periodically extracts least significant bits from receiving side PCM signal code-words and inserts the predetermined bit-pattern sequence into the receiving side PCM signal code-words in place of the extracted bits.

29. The echo canceller of claim 26, wherein said pattern insertion means is arranged between exchanges of a channel associated signaling system and periodically extracts second bits from least significant bits of receiving side PCM signal code-words and inserts the predetermined bit-pattern sequence into the receiving side PCM signal code-words in place of the extracted second bits.

30. The echo canceller of claim 26 wherein said pattern insertion means is arranged between exchanges of a channel associated signaling system and extracts least significant bits of receiving side PCM signal code-words with a period which has a prime number relation with a period of a bit robbing PCM signaling system and inserts the predetermined bit-pattern sequence into the receiving side PCM signal code-words in place of the extracted bits.

31. The echo canceller of claim 26, further comprising pattern detecting means for detecting said predetermined pattern in a transmitting side input PCM signal.

32. The echo canceller of claim 26, further comprising:

(4) signaling detection means for determining whether a voice channel is in idle state; and

(5) pattern control means for controlling said pattern insertion means so that said pattern is inserted when said signaling detection means detects a change from busy state to idle state;

wherein said operation control means disables said echo cancelling means until said signaling detection means detects the status change from busy state to idle state.

33. The echo canceller of claim 32, wherein said signaling detection means includes channel associated signaling detection means.

34. The echo canceller of claim 32, wherein said signaling detecting means includes common signaling detection means.

35. The echo canceller of claim 32, wherein said signaling detecting means includes an input port indicating the signaling status of the voice channel.

36. The echo canceller of claim 35, further comprising:

(6) tone detecting means for determining when a disable tone is present;

(7) pattern control means for disabling said pattern insertion means when said disable tone is present; and

wherein said operation control means disables said echo cancellation circuit when said disable tone is present.

37. The echo canceller of claim 36, wherein said tone detecting means includes an input port for indicating modem signal detection.

38. The echo canceller of claim 36, wherein said tone detecting means includes means for detecting a disable tone by monitoring said transmitting side and receiving side PCM signals.

39. The echo canceller of claim 38, wherein said disable tone is a modem answering tone.

40. The echo canceller of claim 26, further comprising:

(4) second pattern detecting means for detecting said pattern by monitoring a receiving side input PCM signal; and

(5) pattern disabling means for disabling said pattern insertion means in response to pattern detection by said second pattern detecting means.

41. The echo canceller of claim 40, further comprising:

(6) tone detecting means for determining when a disable tone is present;

(7) pattern control means for disabling said pattern insertion means when said disable tone is present;

wherein said operation control means disables said echo cancellation of the echo cancelling means when said disable tone is present.

42. An echo canceller which cancels echo generated, comprising:

(a) an echo cancelling means for cancelling echo signal;

(b) a pattern inserting means for inserting a predetermined bit-pattern sequence into a receiving side pulse code modulation (PCM) output signal; and

(c) a pattern detecting means for detecting a predetermined bit-pattern by monitoring a transmitting side input PCM signal, and disabling the echo cancelling function of the echo cancelling means when the predetermined bit-pattern is detected.

43. An echo canceller which cancels echo generated, comprising:

(a) an echo cancelling means for cancelling echo signal;

(b) a pattern inserting means for extracting a portion of a receiving side pulse code modulation (PCM) input signal and inserting a predetermined bit-pattern sequence into a receiving side PCM output signal in place of the extracted portion; and

(c) a pattern detecting means for detecting a predetermined bit pattern by monitoring a transmitting side input PCM signal, and disabling the echo cancelling function of the echo cancelling means when the predetermined bit-pattern is detected.

44. A method for controlling echo cancellation in an echo canceller which cancels echo generated, said method comprising the steps of:

(1) inserting a predetermined bit-pattern sequence in a receiving side pulse code modulation (PCM) signal; and

(2) disabling said echo cancellation when the predetermined bit-pattern is detected in a transmitting side input PCM signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an echo canceller which cancels echo generated at 2-wire/4-wire conversion portion due to the impedance mismatching and a method for controlling the echo cancellation of the echo canceller.

2. Description of the Prior Art

Along with the increasing of communication demand in recent years, communications networks, which transmit integrated signals such as voice, images and data signals using digital multiplexers, are constructed everywhere in the world. For the high efficiency transmission of voice signals, it is usual to equip low-bit-rate CODECs with the digital multiplexers.

Furthermore, in case that the transmission delay becomes significant, because of the long transmission path such as international links or because of the processing delay introduced by the voice CODECs, echo cancellers shall also be equipped with the digital multiplexers.

FIG. 13 is an example of a telephone communication network which connects three points using digital multiplexers. In FIG. 10, elements 1, 2 and 3 are telephone terminals, elements 4, 5 and 6 are exchanges, elements 7, 8 and 9 are digital multiplexers, elements 10 and 11 are full duplex transmission links. When at telephone call from telephone terminal 1 to terminal 2, for example, is originated, only one transmission link and a pair of echo canceller, consequently, are involved within the end-to-end path. But when a telephone call from telephone terminal 1 to terminal 3, for example, is originated, two transmission links, 10 and 11, are connected in tandem by the exchange 5 to constitute an end-to-end transmission path, and two pairs of echo cancellers, consequently, are involved within the path as indicated in FIG. 14.

In such circuit configuration, it is desirable to disable the echo cancellers 13 and 14 which are put in the midst of the path. One reason is that the echo canceller which is put in the midst of the transmission path, echo canceller 13 for example, is not always capable to accommodate itself to the unknown transmission delay which exist across the exchange, where the total echo delay consists of the delays introduced by transmission links, link 11 for example, and the processing delays introduced by the CODECs, those implemented within multiplexers 8 and 9 for example. If the signal path across the exchange is composed of plural links, there is the possibility that the sum of the delays exceeds the adaptability of the echo canceller. Another reason is that the plural echo cancellers in the same path which serve for the same telephone terminal, echo canceller 13 and 15 for telephone terminal 1 for example, may disturb their echo cancelling operations to each other because each operation, which results in the modification of the transmission characteristics of the echo path, is performed independently. For those reasons, the echo path estimation of each echo canceller connected in tandem may be interfered and the overall transmission quality for the voice channel may be deteriorated.

In the past, for example, the laid-open Japanese patent publication No. 4-165825/92 discloses an echo canceller which disables echo cancelling function automatically when it is necessary. FIG. 15 shows a block diagram of the echo canceller in the above cited reference.

In FIG. 15, element 17 is an oscillator, element 22 is an echo canceller, elements 19 and 20 are channel associated signalling detectors, element 21 is a switching element which selects the output signal of echo canceller or output signal of oscillator for its output, element 18 is a micro processor which controls the operation of switching element 21 based on the information derived from channel associated signalling detectors 19 and 20, element 23 is a transmitting side input port, element 24 is a receiving side output port, element 25 is a transmission side output port and element 26 is a receiving side input port. The external circuit from element 24 to element 23 is usually called the tail circuit and another external circuit from element 25 to element 26 is usually called the long haul circuit. An echo canceller is designed to cancel the echo signal originated in the tail circuit.

The operation of echo canceller shown in FIG. 15 is as follows. The oscillator 17 generates the 2100 Hz tone signal which corresponds to the echo canceller disabling tone signal as specified in the ITU-T Recommendation G.165. The micro processor 18 senses the status change of channel associated signalling information using the channel associated signalling detectors 19 and 20, and recognizes whether the voice channel is in a busy state or in the idle state. The switch 21 operates under the control of the micro processor 18, normally selecting the input signal from the echo canceller 22 for its output. But for a pre-determined short period after the status change of signalling information from the idle state to the busy state, the switch 21 selects the 2100 Hz tone signal generated by the oscillator 17 for its output. On detecting the 2100 Hz disabling tone, the echo cancellers in the network with automatic disabling capability can disable the echo cancelling function. This invention was made to disable the echo cancelling function within the digital network when the terminal equipment with echo cancelling capability, such as speaker phone terminals or tele-conference terminals are connected to the digital network with echo cancelling capabilities within the network to eliminate the cross interference between the network echo cancelling functions and the echo cancelling function of external terminal equipment.

This invention is valid only when the disabling tone generation capability is implemented with the echo canceller of terminal equipment. If the echo cancellers of above stated invention are implemented within the transmission network, in digital multiplexers for example, there is no way for each echo canceller to recognize whether it is put at the either end of the transmission path, as echo cancellers 12 or 15 in FIG. 13, or put at the midst of the transmission path, as echo cancellers 13 and 14 in FIG. 13 for example. In such case as indicated in FIG. 13 for example, the echo cancelling function of echo cancellers 12 and 15 which have to cancel the echo signals, generated at 4-wire/2-wire conversion elements within the exchanges 4, 6 and also in telephone terminal 1 and 3, are also disabled at the same time and the network performance is deteriorated.

SUMMARY OF THE INVENTION

It is a primary object of the present invention to provide an echo canceller which can disable its echo cancelling function when it is put in the midst of the tandem connection of the transmission links, and the exchange in its tail circuit makes the connection to the tail circuit of another echo canceller, not to the telephone terminal.

Also it is another object of the present invention to provide an echo canceller which limits the degradation of signal-to-noise (S/N) ratio of associated voice channel to realize the above mentioned object and prevents an occurrence of deterioration of voice quality and voice band data errors.

It is further object of the present invention to provide an echo canceller which has a simplified construction in order to realize the object of the invention.

According to one aspect of the invention, the echo canceller which cancels echo generated at 2-wire/4-wire conversion portion due to the impedance mismatching includes: an echo canceller for cancelling echo signal; a bit-pattern inserting means for inserting a predetermined bit-pattern by extracting bits from the receiving side input pulse code modulation (PCM) bit stream; and a bit-pattern detecting means for detecting the predetermined bit-pattern by monitoring the transmitting side input PCM bit stream.

According to another aspect of the present invention, the echo canceller further includes: a first channel associated signalling detection means for determining whether the voice channel at the receiving side is in idle state; a second channel associated signalling detection means for determining whether the voice channel at the transmitting side is in idle state; and an operation controlling means for disabling echo cancelling function of the echo canceller, which initiates disabling on detecting the predetermined bit-pattern and terminates disabling on recognizing both the transmitting and receive side signalling status to be in idle state.

According to another aspect of the present invention, the echo canceller further includes: a first common channel signalling detection means for determining whether the voice channel at the receiving side is in idle state; a second common channel signalling detection means for determining whether the voice channel at the transmitting side is in idle state; and an operation controlling means for disabling echo cancelling function of the echo canceller which initiates disabling on detecting the predetermined bit-pattern and terminates disabling on recognizing both the transmitting and receive side signalling status to be in idle state.

According to another aspect of the present invention, an echo canceller which cancels echo generated at 2-wire/4-wire conversion portion due to the impedance mismatching, where neither channel associated signalling detection nor common channel signalling detection is executed within the echo canceller includes: an echo canceller for cancelling echo signal; a bit-pattern inserting means for inserting predetermined bit-pattern into the receiving voice channel PCM bit stream; a bit-pattern detecting means for detecting the predetermined bit-pattern by monitoring the transmitting side input PCM bit stream; a signalling status information input port for obtaining the signalling information whether the relevant channel is in idle state or in busy state; and an operation controlling means for disabling echo cancelling function of the echo canceller for the duration starting from the detection of predetermined bit-pattern by the bit-pattern detecting portion or while the modem signal is conveyed in the voice channel and also disable to the end of the call which is notified by the signalling status input, and also enables the pattern inserting portion to insert the predetermined bit-pattern into receiving side output PCM signal bit stream.

According to another aspect of the present invention, an echo canceller which cancels echo generated at 2-wire/4-wire conversion portion by impedance mismatching, where voice channel is not coded by low bit rate coding includes: an echo cancelling means for canceling echo signal; a pattern inserting means for inserting a predetermined bit-pattern sequence a first pattern detecting means for detecting a predetermined bit-pattern by monitoring transmitting side input PCM signal bit stream to disable the echo cancelling means; a second pattern detecting means for detecting predetermined bit-pattern sequence by monitoring receiving side input PCM signal bit stream to inhibit bit-pattern insertion at the pattern inserting means.

According to another aspect of the present invention, an echo canceller which cancels echo generated at 2-wire/4-wire conversion portion by impedance mismatching includes; an echo cancelling means for canceling echo signal; a pattern inserting means for inserting a predetermined bit-pattern into the receiving voice channel PCM signal bit stream except for the duration when a data call is applied to the relevant echo canceller, a pattern detecting means for detecting predetermined bit-pattern sequence to disable the echo cancelling function of echo cancelling means; a tone detecting means for detecting a modem answering tone by monitoring a transmitting side and a receiving side input PCM signals; a power detecting means for detecting the end of data call by monitoring the absence of signal powers for more than predetermined period both in transmitting side and in receiving side input PCM signals; and a register the output of which is set for the duration of the data call started by the modem answering tone detection signal of the tone detecting means and terminated by the output of the power detecting means which implies the end of the data call.

According to another aspect of the present invention, an echo canceller which cancels echo generated at 2-wire/4-wire conversion portion by impedance mismatching includes; an echo cancelling means for canceling echo signal; a first channel associated signalling detecting means for determining whether the voice channel at the receiving side is in idle state; a second channel associated signalling detecting means for determining whether the voice channel at the transmitting side is in idle state; a pattern inserting means for inserting a predetermined bit-pattern under the control of operation controlling means a pattern detecting means for detecting a predetermined bit-pattern by monitoring a transmitting side input PCM signal; a tone detecting means for detecting a modem answering tone by monitoring both transmitting side and receiving side input PCM signals; and an operation controlling means for disabling echo cancelling function of the echo cancelling means for the duration starting from the detection of predetermined bit-pattern in the transmitting side input PCM bit stream at the pattern detecting means or from the detection of modem answering tone by monitoring both transmitting side and receiving side input PCM signals at the tone detecting means, until the voice channel becomes idle again by detecting the voice channel status changes from busy state to idle state both in receiving side and transmitting side at the first and the second channel associated signalling detecting means, and for enabling the pattern inserting means for a predetermined duration when the voice channel status changes from idle state to busy state by detecting the signalling status both in receiving side and transmitting side become busy state at the first and the second channel associated signalling detecting means but disabled overridden by the detection of modem answering tone at the tone detecting means.

According to another aspect of the present invention, an echo canceller which cancels echo generated at 2-wire/4-wire conversion portion by impedance mismatching includes; an echo cancelling means for canceling echo signal; a first common channel signalling detecting means for determining whether the voice channel at the receiving side is in idle state; a second common channel signalling detecting means for determining whether the voice channel at the transmitting side is in idle state; a pattern inserting means for inserting a predetermined bit-pattern under the control of operation controlling means; a pattern detecting means for detecting a predetermined bit-pattern by monitoring a transmitting side input PCM signal; a tone detecting means for detecting a modem answering tone by monitoring both transmitting side and receiving side input PCM signals; and an operation controlling means for disabling echo cancelling function of the echo cancelling means for the duration starting from the detection of predetermined bit-pattern in the transmitting side input PCM bit stream at the pattern detecting means or from the detection of modem answering tone by monitoring both transmitting side and receiving side input PCM signals at the tone detecting means until the voice channel becomes idle again by detecting the voice channel status changes from busy state to idle state both in receiving side and transmitting side at the first and the second common channel signalling detecting means, and for enabling the pattern inserting means for a predetermined duration when the voice channel stat