Efficient transmission of digital return path data in cable television return path

• US 20030154498A1
• Filed: 03/19/2002
• Published: 08/14/2003
• Est. Priority Date: 02/12/2002
• Status: Active Grant

• Current Assignee: II-VI Delaware, Inc. (Coherent Corp.)
• Original Assignee: Finisar Corporation (II-VI Incorporated)
• Inventors: Sage, Gerald F.
• Granted Patent: US 7,751,718 B2
• Time in Patent Office: Days
• US Class Current: 725/131
• CPC Class Codes: H04N 21/437 Interfacing the upstream path of the transmission network, e.g. for transmitting client requests to a VOD server flow control in data networks H04L47/10; streaming protocols, e.g. RTP or RTCP, H04L65/65; scheduling or organising the servicing of application requests in data packet switching networks H04L67/60
• CPC Class Codes: H04N 21/6168 involving cable transmission, e.g. using a cable modem
• CPC Class Codes: H04N 7/17309 Transmission or handling of upstream communications

Abstract

An efficient means for transmitting digitized return path signals over a cable television return path is disclosed. In one embodiment of the invention, the cable television return path includes a node that receives an analog return signal from a subtree of the cable television system and generates a digital transport signal representative of the analog return path signal. The digital transport signal, however, is not a digitized form of the analog return signal. Rather, the digital transport signal is encoded such that fewer bits are used to represent the analog return signal without substantially impacting the accuracy and dynamic range of the signal. At the hub, the digital transport signal is decoded and converted to produce an analog signal that is a close approximation of the analog return signal.

Claims

1. A signal encoder for use in a cable television return path, comprising:

an input configured to receive an N-bit digital input sample from an analog-to-digital converter, the N-bit digital input sample having N bits;

logic configured to determine whether the N-bit digital input sample has an amplitude that is larger than a predefined threshold, the logic further configured to generate at least one selection output bit indicative of whether the amplitude is larger than the predefined threshold; and

logic configured to produce a digital output sample representative of the N-bit digital input sample in response to the at least one selection output bit, the digital output sample having one of a first subset and a second subset of the N bits, the digital output sample having the at least one selection output bit but having fewer bits than N; and

an output, coupled to the logic, configured to provide the digital output sample to an optical transmitter.

2. The signal encoder of claim 1 wherein the predefined threshold equals to a maximum value of the analog-to-digital converter divided by four.

3. The signal encoder of claim 2, wherein the first subset includes N−

2 most significant bits of the N-bit digital input sample and wherein the second subset includes N−

3 least significant bits of the N-bit digital input sample.

4. The signal encoder of claim 1 wherein the first subset includes at least one bit not included in the second subset and the second subset includes at least one bit not include in the first subset.

5. A signal decoder for use in a cable television return path, comprising:

an input configured to receive an M-bit digital sample from an optical receiver, the M-bit digital sample having M bits that includes at least one selection bit and non-selection bits;

logic configured to insert a plurality of padding bits with the non-selection bits to produce a digital output sample representative of the M-bit digital sample, the digital output sample having more bits than the M-bit digital sample; and

a digital-to-analog converter coupled to the logic, the digital-to-analog converter configured to provide an analog signal corresponding to the digital output sample.

6. The decoder of claim 5 wherein the logic is configured to insert the padding bits as the least significant bits of the digital output sample when the at least one selection bit indicates that the M-bit digital sample is a large sample.

7. The decoder of claim 5 wherein the logic is configured to extend a sign bit of the non-selection bits to produce the digital output sample when the at least one selection bit indicates that the M-bit digital sample is a small sample.

8. A cable television return path of a cable television system, comprising:

an analog-to-digital converter configured to receive an analog return signal from a subtree of the cable television system and configured to generate a first digital signal representative of the analog return signal, the first digital signal having a stream of first digital samples each having N bits;

an encoder configured to receive the first digital signal and to generate a second digital signal that includes a stream of second digital samples each having a fewer number of bits than N, each of the second digital samples including one of a first subset of most significant bits and a second subset of least significant bits of a corresponding one of the first digital samples;

an optical transmitter configured to convert the second digital signal into an optical signal;

an optical receiver configured to receive the optical signal and to convert the optical signal into a replica of the second digital signal;

a decoder configured to recover a third digital signal from the replica of the second digital signal, the third digital signal including a stream of third digital samples, wherein each of the third digital samples includes N bits; and

a digital-to-analog converted configured to convert the third digital signal to an analog signal that is substantially equivalent to the analog return signal.

9. The cable television return path of claim 8, wherein the encoder comprises:

an input configured to receive a digital sample of the first digital signal from the analog-to-digital converter;

logic configured to determine whether the digital sample has an amplitude that is larger than a predefined threshold, the logic further configured to generate at least one selection output bit indicative of whether the digital sample is larger than the predefined threshold; and

logic configured to produce a digital output sample representative of the N-bit digital input sample in response to the at least one selection output bit, the digital output sample having one of a first subset and a second subset of the N bits, the digital output sample having the at least one selection output bit but having fewer bits than N.

10. The cable television return path of claim 9, wherein the predefined threshold equals to a maximum value of the analog-to-digital converter divided by four.

11. The cable television return path of claim 10, wherein the first subset includes N−

2 most significant bits of the N-bit digital input sample and wherein the second subset includes N−

3 least significant bits of the N-bit digital input sample.

12. The cable television return path of claim 8, wherein the decoder comprises:

an input configured to receive a digital sample from an optical receiver, the digital sample having fewer bits than N but includes non-selection bits and at least one selection bit; and

logic configured to insert a plurality of padding bits with the non-selection bits to produce a digital output sample representative of the M-bit digital sample, the digital output sample having N bits.

13. The cable television return path of claim 12 wherein the decoder is configured to insert the padding bits as the least significant bits of the digital output sample when the at least one selection bit indicates that the digital sample is a large sample.

14. The cable television return path of claim 12 wherein the decoder is configured to extend a sign bit of the non-selection bits to produce the digital output sample when the at least one selection bit indicates that the digital sample is a small sample.