Experiment at higher frame rates

Forum for discussion of narrow-bandwidth mechanical television

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Re: Sample rates.

Postby gary » Fri May 18, 2007 2:40 pm

Steve Anderson wrote:
Well, you've made up my mind. I have been dithering over the past few days as to what sample rate to use for the .wav files I intend to create. Now it's a very definate 48kHz.

I often wondered how PCs generated some of these 'awkward' frequencies, like 44.1kHz, I wonder why it was chosen in the first place? Wouldn't 44kHz (exact) been easier. I guess Sony and Philips had their reasons.

48kHz means we get an exact 400Hz line rate on playback, plus an easier samples per line number of 120. It does sadly make the files 9% larger.

Steve A.


From John Watkinson, The Art of Digital Audio, 2nd edition, pg. 104:

In the early days of digital audio research, the necessary bandwidth of about 1 Mbps per audio channel was difficult to store. Disk drives had the bandwidth but not the capacity for long recording time, so attention turned to video recorders. These were adapted to store audio samples by creating a pseudo-video waveform which would convey binary as black and white levels. The sampling rate of such a system is constrained to relate simply to the field rate and field structure of the television standard used, so that an integer number of samples can be stored on each usable TV line in the field. Such a recording can be made on a monochrome recorder, and these recording are made in two standards, 525 lines at 60 Hz and 625 lines at 50 Hz. Thus it is possible to find a frequency which is a common multiple of the two and is also suitable for use as a sampling rate.

The allowable sampling rates in a pseudo-video system can be deduced by multiplying the field rate by the number of active lines in a field (blanking lines cannot be used) and again by the number of samples in a line. By careful choice of parameters it is possible to use either 525/60 or 625/50 video with a sampling rate of 44.1KHz.

In 60 Hz video, there are 35 blanked lines, leaving 490 lines per frame or 245 lines per field, so the sampling rate is given by :

60 X 245 X 3 = 44.1 KHz

In 50 Hz video, there are 37 lines of blanking, leaving 588 active lines per frame, or 294 per field, so the same sampling rate is given by

50 X 294 X3 = 44.1 Khz.

The sampling rate of 44.1 KHz came to be that of the Compact Disc. Even though CD has no video circuitry, the equipment used to make CD masters is video based and determines the sampling rate.

=========================================

Choosing 48 (or 32) kHz is fine provided the wave file is not going to be burnt to an audio CD because you can't trust the conversion algorithms of the burning software to not distort the video signal.
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Postby Klaas Robers » Sat May 19, 2007 6:40 am

This is just part of the story.

In the years that CD was developed by my collegues of Philips and the people of sony, this last company had a set-next box on the market, belonging to their Betamax video recorder, that digitized an audio signal, converted this to a video signal that could be recorded on this low definition video recorder, and could also do the opposite process, i.e. play back. Because of the said limitations indeed a sampling frequency of 44.100 Hz was choosen, which is not an unlogical number, because 44.100 = 2^2 . 3^2 . 5^2 . 7^2, so the first four primes squared. There was not sufficient bandwith for 48 kHz at 2 x 16 bits per stereo sample.

Then in the discussions about the CD between Philips and sony my collegues suggested 48 kHz as a sampling frequency, because this was already a professional standard. But the sony-guys wanted to have the longest playing duration (there are also stories to be told about that item) and/or the smallest disc. That is why they suggested 44.100Hz, a frequency that they knew already, and of which they knew how to solve the problems in A/D and D/A. They then sugested to standardise ONLY the disc parameters, so they needed not to discover the details of the technical solutions on the problems to the crew of Philips. They could honestly say that the problems were solvable, look to our set-next box.

Why a set-next box, well the Betamax VCR was a top-loader, so you could not place a box on top of it. Otherwise it would surely have been called a set-top box.
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44.1kHz.

Postby Steve Anderson » Sat May 19, 2007 1:06 pm

Thanks Gents,

You've filled in a large black hole in my knowledge regarding the history of 44.1kHz.

I remember the equipment you mention, a friend of mine had one made by Sony, sometime in the late 70s I think, a PCM-something-something-something from memory.

If you 'watched' the PCM audio on a TV it looked like wobbling barcodes. Quite mesmerizing, along with the 'far out' music of the times!

Steve A.
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Postby Steve Anderson » Sat May 19, 2007 1:14 pm

Choosing 48 (or 32) kHz is fine provided the wave file is not going to be burnt to an audio CD because you can't trust the conversion algorithms of the burning software to not distort the video signal.


That indeed is one of the reasons I have been undecided about a sample rate.

Steve A.
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Postby AncientBrit » Mon May 21, 2007 11:11 pm

Steve,

Are you intending the NBTV signal to be phase locked to the sample rate?

That might be possible with an electronic source of NBTV pix, but mechanical sources are all over the place.

You would need some form of TBC (timebase correction) to be included in the signal path for a mechanical source.

The other possiblilty is to have a gated NBTV line locked conversion fed into a FIFO with padding of the samples to bring it to the required 44.1kHz rate.

Regards,

GL
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Sample rates.

Postby Steve Anderson » Tue May 22, 2007 12:56 pm

Are you intending the NBTV signal to be phase locked to the sample rate? That might be possible with an electronic source of NBTV pix, but mechanical sources are all over the place.


Yes, I am creating files where the sample rate (48kHz) is an exact interger multiple of the 400Hz NBTV line rate. This will provide a clean stable waveform to get a newly constructed display going.

After that checks should be done to see what errors it can handle, will it lock to 350Hz or 450Hz? Throw in some noise/ripple/hum, and see how it copes.

To that end I built a NBTV TSG (Test Signal Generator) some years ago. The 'scope displays below show the 'clean' signal and the 'dirty' version. If your monitor locks and displays this you're doing really well!

I started writing an item for the newsletter a while back about using TBC techniques to clean up mechanical irregularities in the signal, but I didn't complete it.

Steve A.
Attachments
spg1.jpg
spg1.jpg (74.73 KiB) Viewed 3578 times
Clean.jpg
Clean.jpg (65.95 KiB) Viewed 3578 times
Dirty.jpg
Dirty.jpg (74.52 KiB) Viewed 3578 times
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Postby Klaas Robers » Tue May 22, 2007 10:00 pm

If you observe the video signals of the NBTV-CD number 3, then you see that NBTV-video is inserted of the mechanical camera of Eddy Greenhough, Pedro,s CD-disc camera and the flying spot scanner built by Jeremy Jago. I can tell you that these signals were not at all cristal stable. Especially Eddy Greenhoughs camera, which had no sync, needed a lot of processing before I could add sync. So a lot of timebase correction has been done in my computer (all in software).

Anyway, all signals on this disc are timebase corrected and coupled to the CD-sample rate, even the mayor part of the video, coming from the scanconverter of Peter Smith. This was cristal stable, but not locked to the samplerate of the CD and once you made the software to do TBC there is no reson not to do this for certain signals.

In CD no1 this is not done and this prepared me quite a lot of problems on editing the video.
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Postby AncientBrit » Tue May 22, 2007 10:21 pm

Steve,

Any chance of that TBC article seeing the light of day?

I for one would like to read it,

Cheers,

GL
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Postby AncientBrit » Tue May 22, 2007 10:21 pm

Steve,

Any chance of that TBC article seeing the light of day?

I for one would like to read it,

Cheers,

GL
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Postby AncientBrit » Tue May 22, 2007 10:28 pm

Sorry about the "stutter", in future I won't use the "Back" key which has the effect of re-posting,

Regards,

GL
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