Method for converting a plurality of signals from one of a digital and analogue form to the other form

• US 6,621,434 B2
• Filed: 06/10/2002
• Issued: 09/16/2003
• Est. Priority Date: 06/11/2001
• Status: Active Grant

• Current Assignee: Analog Devices, Inc.
• Original Assignee: Analog Devices, Inc.
• Inventors: Barry, Joseph Michael, Cotter, Martin Gerard
• Primary Examiner(s): Tokar, Michael
• Assistant Examiner(s): Mai, Lam
• Application Number:
  US10/166,439
• Publication Number:
  US 20030052804A1
• Time in Patent Office: 463 Days
• Field of Search: 341/143, 341/141, 375/242, 375/147, 375/240.01, 375/240.02, 370/369, 370/503, 386/55, 386/111, 386/64
• US Class Current: 341/141
• CPC Class Codes: H04N 5/14 Picture signal circuitry for video frequency region cameras or camera modules comprising electronic image sensors, or control thereof H04N23/00
• CPC Class Codes: H04N 7/0102 involving the resampling of the incoming video signal
• CPC Class Codes: H04N 7/0125 one of the standards being a high definition standard

Abstract

A method for selectively and sequentially converting digital video data signals in three formats, namely, standard definition, progressive scan and high definition formats to analogue form such that the same analogue reconstruction filter can be used for filtering the analogue forms of the signals after conversion. The digital signals prior to being converted are over-sampled at respective over-sampling frequencies for displacing image frequencies to frequencies sufficiently spaced apart from the widest band width signal so that the analogue reconstruction filter can be provided for filtering the widest band width signal as well as the other band width signals and also can be provided with a relatively low roll-off rate of attenuation. The standard definition video data signals are over-sampled at eight times the nyquist sampling frequency, and the progressive scan signals are over-sampled at four times the nyquist sampling frequency, thus displacing image frequencies of the standard definition and progressive scan signals to 108 MHz, so that the centre frequency of the image frequency is displaced to 108 MHz. The high definition video signals are over-sampled at twice the nyquist sampling frequency, thus displacing the image frequency of the high definition signal to 148.5 MHz, so that its centre frequency is displaced to 148.5 MHz. This, thus, leaves a frequency band gap of approximately 66 MHz between the high definition band width signal and the closest image frequency, namely, that of the progressive scan signal. The method can also be applied to the conversion of analogue video signals to digital form whereby the same analogue anti-aliasing filter can be used for the analogue signals in the three formats.

Claims

1. A method for selectively and sequentially converting a plurality of signals of different frequency band width from one of a digital and analogue form to the other of the digital and analogue form, wherein the signals in the analogue form are passed along a common analogue path through an analogue filter, the method comprising the steps of:

sequentially selecting the signals to be converted,

sequentially passing the selected signals through a converter circuit for converting the signals from the one of the digital and analogue form to the other of the digital and analogue form, and

over-sampling the respective signals in the converter circuit at respective over-sampling frequencies so that the analogue forms of the signals can be filtered in the same analogue filter in the common analogue path.

2. A method as claimed in claim 1 in which the signal of widest band width is over-sampled at the lowest over-sampling rate.

3. A method as claimed in claim 1 in which the signal of narrowest band width is over-sampled at the highest over-sampling rate.

4. A method as claimed in claim 1 in which the signal of widest band width is over-sampled at a frequency of at least twice the nyquist sampling frequency.

5. A method as claimed in claim 1 in which the signals are digital signals, and the converter circuit is a digital to analogue converter circuit for converting the signals from digital form to analogue form and the analogue filter is a reconstruction filter.

6. A method as claimed in claim 5 in which the over-sampling frequencies for the respective signals are selected such that image frequencies in the analogue signal are sufficiently displaced from the widest frequency band width signal that the image frequencies can be removed from each of the analogue signals by the same analogue filter without affecting the signals.

7. A method as claimed in claim 6 in which the over-sampling frequencies for the respective signals are selected such that the image frequency, the lowest frequency of which is closest to the widest band width signal is displaced from the widest band signal to a frequency at least twice the band width of the widest band width signal.

8. A method as claimed in claim 6 in which the over-sampling frequencies for the respective signals are selected such that the image frequency, the lowest frequency of which is closest to the widest band width signal is displaced from the widest band signal to a frequency at least three times the band width of the widest and width signal.

9. A method as claimed in claim 1 in which the signals are in analogue form and are converted to digital form, the analogue filter being an anti-aliasing filter.

10. A method as claimed in claim 9 in which the over-sampling frequencies at which the respective signals are over-sampled are selected such that the analogue anti-aliasing filter can be such as to preserve the signal of widest band width and reject signals of frequencies above the lowest sampling frequency used less the widest band width of the signal sampled at the lowest sampling frequency.

11. A method as claimed in claim 10 in which unwanted frequencies between the band width of each signal and half the frequency at which it is over-sampled are removed by a combined anti-aliasing filter and decimation filter.

12. A method as claimed in claim 1 in which the over-sampling frequencies are selected such that the analogue filter can be provided with a low roll-off rate of attenuation.

13. A method as claimed in claim 1 in which the band width of a first one of the signals is in the range of 4 MHz to 8 MHz, and is over-sampled at a rate of eight times the nyquist sampling frequency.

14. A method as claimed in claim 13 in which the first signal is of band width of approximately 6 MHz.

15. A method as claimed in claim 1 in which the band width of a second one of the signals is in the range of 10 MHz to 18 MHz, and is over-sampled at a rate of four times the nyquist sampling frequency.

16. A method as claimed in claim 15 in which the second signal is of band width of approximately 12 MHz.

17. A method as claimed in claim 1 in which the band width of a third one of the signals is in the range of 25 MHz to 35 MHz, and is over-sampled at a rate of twice the nyquist sampling frequency.

18. A method as claimed in claim 17 in which the third signal is of band width of approximately 30 MHz.

19. A method as claimed in claim 1 in which the signals are video signals.

20. A method as claimed in claim 19 in which the first signal is in standard definition format.

21. A method as claimed in claim 19 in which the first signal is in progressive scan format.

22. A method as claimed in claim 19 in which the first signal is in high definition format.

23. A method as claimed in claim 1 in which the analogue filter is a second or higher order filter.