Troubleshooting Hedghog

[Terry]

it will work for audio but 12 bits aren't really enough for high quality.

What a pity that 405-line sets don't come equipped with NICAM decoders - you'd have two bits going spare!

[ppppenguin]

Terry, I'll do a 405 "sound in syncs" if you're not careful

AFAIK, SIS was only done on 625 and was never broadcast. Do you know if there was ever an accident where it escaped? My only involvement with SIS was to design a SIS stripper, to get rid of SIS and produce a clean video waveform.

https://www.bbc.co.uk/rd/publications/rdreport_1969_35
https://en.wikipedia.org/wiki/Sound-in-Syncs

SIS was a brilliant invention. Saved the cost of sound circuits from the studio to the TX, made sure that sound and vision stayed together and gave improved quality compared to analogue sound circuits.

[Mark Hennessy]

The original PCM radio distribution was 13 bits. More than adequate. 12 bits easily gets you below 0.1% THD+N

Dedicated audio ADCs oversample and digitally filter. Makes for easier implementation, but is probably not what you want for a general-purpose ADC on an FPGA dev board. It goes to pretty high sample rates, so oversampling would be feasible if needed, but somehow I suspect that 12 bits is generous for a telly...

[ppppenguin]

13 bit PCM was good enough for audio with well controlled levels which was going to be utilised by FM transmitters. If correctly dithered to avoid unpleasant effects 13 bits gives you about 80dB s/n ratio which is better than an FM transmission. It was also a lot better than the analogue circuits previously used from the studios to the transmitters. Getting stereo beyond London and Wrotham was a horrible business before PCM. I can remember Angus McKenzie writing in Hifi News about the BBC's failure to get decent stereo even as far as Birmingham.

Required number of bits is always an interesting debate. Assuming you use dithering correctly, it'ssimply a matter of what signal to noise ratio you want to achieve. Roughly 6dB per bit. If audio levels are not accurately controlled then you need more bits. My understanding is that 16 bits is fine for controlled levels (think CD) while in the studio you want at least 20. In practice I suspect that a lot of modern audiio kit uses 32 bt arithmetic simply because it costs nothing. Audio ADCs can get to 20 bit accuracy with some certainty. Any more than that is probably wishful thinking.

[ppppenguin]

This is the first use of the Motorola modulator chips for System A that I know of. David Robinson designed his colour 625 to 405 converter c2005. He used a PIC to program the modulators.

For colour coding he used a commercially available PAL/NTSC coder chip and persuaded it to do 405 NTSC. I had actually tried and failed to do this myself. These days it's just so much easier to do the NTSC coding in the FPGA, as Darryl did in the colour version of the SCRF.

[Freya]

More about it here https://www.vintage-radio.net/forum/show...php?t=1940

[Murphyv310]

That's an old thread now with all the links broken...... pity.

[`FRANK.C]

The post that Freya has linked to is where I first seen the modulator.
The modulator in Hedghog is almost the same with some minor changes. The audio buffers and video filter are removed.
The FPGA provides a 4 MHz clock which replaces the 4 MHz crystals and the FPGA programs the modulator chips.


Frank

[ppppenguin]

David Robinson's use of the those modulator chips comfortably pre-date's Darryl's use of them in the Aurora SCRF. I don't know if Darryl looked at David's work or had the same ideas himself. As in the Hedghog, the SCRF derives the 4MHz and programming from the FPGA. This is both sensible and economical.

[ppppenguin]

It seems that several people have been buying FPGA cards from China (Aliexpress) that have turned out to be faulty. Might be better to buy from a decent UK supplier with a reputation to protect. You'll pay a few ££ more but get quick delivery and better after sales service if they turn out to be faulty. Getting your money back from Aliexpress is all very well but waiting another few weeks for delivery isn't good. And if you've been caught for import duty etc (normally only if buying more than 1) you'd be struggling to get that back from HMRC.

This could be a reasonable UK possibility:
https://hobbycomponents.com/altera/819-a...-dev-board