Method and system for enhancing digital video transmission to a set-top box

• US 6,546,557 B1
• Filed: 10/13/1998
• Issued: 04/08/2003
• Est. Priority Date: 10/13/1998
• Status: Expired due to Term

• Current Assignee: ARRIS Enterprises LLC (CommScope Holding Company, Inc.)
• Original Assignee: General Instrument Corporation (CommScope Holding Company, Inc.)
• Inventors: Ovadia, Shlomo
• Primary Examiner(s): Faile, Andrew
• Assistant Examiner(s): BUI, KIEU OANH T
• Application Number:
  US09/170,852
• Time in Patent Office: 1,638 Days
• Field of Search: 725/119, 725/122, 725/124, 725/125, 725/131, 725/139, 725/151, 725/107, 725/129, 348/192, 348/193, 375/261, 375/264
• US Class Current: 725/129
• CPC Class Codes: H04N 7/22 Adaptations for optical transmission

Abstract

A method and system for enhancing digital video transmission to set-top boxes improves the performance of a hybrid analog and digital video transmission system, for example, an AM-VSB QAM video transmission system, by selecting a digital channel map based on the relative magnitude and frequency locations of CSO and CTB distortions and the analog channel frequency plan. A variable interleaver may also be implemented in the video transmission system to further reduce the CSO and CTB distortions.

Claims

1. A method to improve the performance of hybrid analog and digital video transmission systems comprising the step of:

determining the relative magnitude and frequency locations of nonlinear distortions;

identifying an analog channel frequency plan used in said transmission system; and

selecting a digital channel map based on said determining step and said identifying step,

wherein a frequency band of a first digital channel is selected in said selecting step such that dominant linear distortions fall outside said frequency band.

2. A method in accordance with claim 1, wherein the method is applied to multiple contiguous digital channels, and wherein the nonlinear distortions fall in gaps between the multiple contiguous digital channels.

3. A method in accordance with claim 1, further comprising the step of downloading the selected digital channel map to the set-top box.

4. A method in accordance with claim 1, wherein the digital channel map selected in said selecting step downshifts each channel center frequency to reduce the nonlinear distortions.

5. A method in accordance with claim 1, wherein the hybrid analog and digital video transmission system is a multichannel AM-VSB/QAM system.

6. A method in accordance with claim 5, wherein the nonlinear distortions determined in said determining step are CSO and CTB distortions.

7. A method in accordance with claim 6, wherein the digital channel map selected in said selecting step downshifts each QAM channel center frequency to reduce the magnitude of the CSO and CTB distortions.

8. A method in accordance with claim 6, wherein said determining step determines the relative magnitude and frequency locations of CSO and CTB distortions based on a type of laser transmitter in the video transmission system.

9. A method in accordance with claim 8, wherein the laser transmitter of the video transmission system is a Directly Modulated (DM) laser transmitter.

10. A method in accordance with claim 8, wherein the laser transmitter of the video transmission system is an Externally Modulated (EM) laser transmitter.

11. A method according to claim 5, wherein a QAM modem in said video transmission system includes an interleaver.

12. A method according to claim 11, wherein the depth of the interleaver is variable.

13. A method according to claim 12, wherein the depth of the interleaver may be adjusted based on the CSO and CTB distortions determined in said determining step.

14. A method according to claim 12, wherein increasing the depth of the interleaver reduces the magnitude of the CSO and CTB distortions.

15. A method in accordance with claim 1, wherein said identifying step identifies a harmonic related carrier (HRC) channel frequency plan.

16. A method in accordance with claim 15, wherein the digital channel map selected downshifts each QAM channel center frequency by 3-MHz relative to the picture carrier frequency.

17. A method in accordance with claim 1, wherein said identifying step identifies an interval related carrier (IRC) channel frequency plan.

18. A method in accordance with claim 17, wherein when said determining step determines that the CSO distortions are dominant, the digital channel map selected downshifts each QAM channel center frequency by 1.75-MHz relative to the picture carrier frequency.

19. A method in accordance with claim 17, wherein when said determining step determines that the CTB distortions are dominant, the digital channel map selected downshifts each QAM channel center frequency by 3-MHz relative to the picture carrier frequency.

20. A method according to claim 1, wherein continuous wave (CW) carriers are used to simulate modulated video carriers.

21. A method according to claim 20, wherein Automatic Gain Circuitry (AGC) maintains an average RF input power at a fixed level.

22. A method according to claim 21, wherein a laser transmitter of the video lightwave transmission system is a Directly Modulated (DM) laser transmitter.

23. A method according to claim 21, wherein a laser transmitter of the video lightwave transmission system is an Externally Modulated (EM) laser transmitter.

24. A method according to claim 21, further comprising the steps of:

a first measuring step for measuring AM CNR and CSO/CTB distortions a first time with modulated video carriers and 0-dBm optical power;

reducing the average RF input level to the laser transmitter; and

a second measuring step for measuring AM CNR and CSO/CTB distortions a second time.

25. A method according to claim 24, wherein an optimum operating point of the laser transmitter for modulated signals is obtained by said second measuring step.

26. A method in accordance with claim 1, further wherein a frequency location of the first digital channel is located above a last analog channel frequency.

27. A method to improve the performance of a multichannel AM-VSB/QAM video lightwave transmission system comprising the steps of:

determining the relative magnitude and frequency locations of CSO and CTB distortions;

identifying an analog channel frequency plan used in said transmission system;

selecting a digital channel map based on said determining step and said identifying step, wherein a frequency band of a first digital channel is selected in said selecting step such that dominant linear distortions fall outside said frequency band; and

adjusting the depth of a variable interleaver or said video lightwave transmission system to reduce the CSO and CTB distortions determined in said determining step.

28. A method in accordance with claim 27, wherein the digital channel map selected downshifts each QAM channel center frequency to reduce the magnitude of the CSO and CTB distortions.

29. A method in accordance with claim 27, wherein increasing the depth of the variable interleaver reduces the magnitude of the CSO and CTB distortions.

30. A method in accordance with claim 27, further wherein a frequency location of the first digital channel is located above a last analog channel frequency.

31. A hybrid analog and digital video transmission system for enhancing digital video transmission, the headend of the system comprising:

means for generating a composite analog, digital and/or data signal; and

means for transferring the generated signal,

wherein said transferring means determines the relative magnitude and frequency locations of nonlinear distortions, and

further wherein said transferring means identifies an analog channel frequency plan used in said transmission system, and

still further wherein a digital channel map is selected based on the determination and identification made by said transferring means, wherein a frequency band of a first digital channel is selected such that dominant linear distortions fall outside said frequency band.

32. A method in accordance with claim 31, further wherein a frequency location of the first digital channel is located above a la,t analog channel frequency.