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In Reply to: Audibility of group delay posted by op48no1 on April 28, 2012 at 07:44:53:
It is possible however to build a filter that has no significant group delay. It's called a "transversal filter" or "sin cosine equalizer".I first came across these in the FM head preamplifiers of analog broadcast video recorders. At the broadcast quality level you can't have any group delay in the FM output from the playback chain. You know what group delay can do to FM! However the tape head outputs follow the same 6db/octave curve as any tape recorder and must still be equalized flat from 5mhz to 10mhz in this case. A conventional equalizer has too much group delay. (Although industrial and consumer VTRs used them anyway. But here there was much lower FM bandwidth. Some as little as 1mhz deviation with 4mhz carrier).
The transversal filter has delay lines that add the delayed signal back to the un-delayed signal and thus correct the roll off.
Now in ANALOG audio the delays required to build a filter or equalizer like this were beyond practical. But today with DIGITAL audio, DSPs, and cheap memory transversal filters for audio are quite common.
BTW, why FM in a video recorder?
Well it's IMPOSSIBLE TO RECORD BASE BAND VIDEO ON TAPE! Audio has a spread of 10 octaves where standard 525 line video has a spread of 17 octaves. Even with our analog technology today you can't build an analog equalizer with a 17 octave spread to flatten a 6db per octave magnetic tape response. No way! So the video signal was FM modulated to compress the bandwidth. The playback FM was then demodulated back to base band video. So while you can't record a bandwidth of 10hz to 4mhz on magnetic tape, you can record easily a bandwidth of 5mhz to 10mhz. Same baseband signal bandwidth but now less octave spread. Of course a high head to tape speed of 1000ips is still needed and that's why rotating the heads.I just though a review of some interesting practical technology here would be refreshing among all this magic wire talk.
Edits: 04/29/12 04/29/12
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Follow Ups
- Transversal Filters - gusser 10:20:33 04/29/12 (2)
- RE: Transversal Filters - rage 12:38:58 04/30/12 (1)
- RE: Transversal Filters - gusser 16:42:51 04/30/12 (0)
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