A digital tone generator includes a modulo adder, an increment number register, two latches and a master clock. The adder continuously adds one of a plurality of predefined increment numbers with the adder's previously generated sums to create further sums and a carry output corresponding to one of a plurality of predetermined output tone frequencies. The system characteristics and increment numbers are selected to correspond to the predetermined frequencies and generate exact tonal frequency signals. The system characteristics and increment numbers are defined using a formula which accounts for the least common multiple of the rational number representation of the predetermined frequencies. In the preferred embodiment, the digital tone generator is used to select and generate one of a plurality of time code tones.
This is a continuation of application Ser. No. 115,181, filed on Aug. 30, 1993, for a DIGITAL TONE OSCILLATOR FOR CERTAIN EXACT FREQUENCIES AND METHOD FOR GENERATING TONES, now abandoned.
Methods and apparatus for variable-rate down-sampling filters for discrete-time sampled systems using a fixed sampling rate are disclosed. The Variable Rate Down-Sampling Filter allows a continuous range of sample rates to be derived from input samples at a fixed rate. The output rate does not have to be related to the fixed input rate in any integral or rational way, and in fact, the output rate can vary in time such as will occur when tracking a signal received from a station using a different timebase. A fixed sampling rate at the A/D Converter greatly simplifies the design of the analog front end. A single anti-aliasing filter can be designed and precisely matched to the fixed sampling rate. Used with a frequency-modulated numerically controlled oscillator (NCO), the Down-Sampling Filter keeps the entire frequency-synthesis and time-tracking loop in the digital domain. The need for an analog time-tracking loop to adjust the sampling instant at the A/D converter is eliminated. The need for an analog frequency synthesizer to generate a variable sampling rate is also eliminated. System design and performance analysis are simplified as well by eliminating the complication of hybrid analog-digital control loops. Details are disclosed.
A node (200) for receiving analog signals transmitted within a broadband communication system (100) includes a summer (205) for summing the analog signals to generate a summed analog signal and an analog-to-digital (A/D) converter (210) coupled to the summer (205) for converting the summed analog signal into a digital electrical signal. The node (200) also includes a digital signal processor (DSP) (220) coupled to the A/D converter (210) for filtering the digital electrical signal in accordance with a filter algorithm to generate a filtered digital signal. The DSP (220) including a control port for receiving a control signal (215) indicative of the filter algorithm that is to be used. The filtered signal is then transmitted upstream within the system (100) by a transmitter (240) coupled to the DSP (220).
A cable television system (100) for transmitting information in forward and reverse directions includes forward and reverse paths over which information is transmitted and a reverse path transmitter (305) and a reverse path receiver (405), both located in the reverse path. The reverse path transmitter (305) receives an analog electrical signal and an external digital signal from an external source, such as a PCS terminal or router. The transmitter (305) then generates a single digital bit stream from the external digital signal and the digital electrical signal and transmits the single digital bit stream modulated at a first wavelength over a fiber optic communication medium (110) that couples the transmitter (305) and the reverse path receiver (405). The reverse path receiver (405) receives and demodulates the single digital bit stream to regenerate the digital electrical signal and the external digital signal, converts the digital electrical signal to the analog electrical signal for processing within the cable television system (100), and forwards the external digital signal for processing by a second external source outside the cable television system (100).
A cable television system (100) having forward and reverse paths includes, in the reverse path, a digital optical transmitter (200) for receiving an RF signal, converting it to a digital signal, and adding a digital pilot tone thereto. A laser is driven in accordance with the summed digital signal to generate a digital optical signal representative of the pilot tone and the RF signal. The cable television system (100) also includes an optical receiver (305) for receiving the digital optical signal and recovering therefrom the RF signal and the pilot tone. The optical transmitter (200) and receiver (305) are coupled by fiber optic communication media (110).
A cable television system (100) having forward and reverse paths includes, in the reverse path, a digital optical transmitter (200) for receiving an RF signal, converting it to a digital signal, and adding a digital pilot tone thereto. A laser is driven in accordance with the summed digital signal to generate a digital optical signal representative of the pilot tone and the RF signal. The cable television system (100) also includes an optical receiver (305) for receiving the digital optical signal and recovering therefrom the RF signal and the pilot tone. The optical transmitter (200) and receiver (305) are coupled by fiber optic communication media (110).
