Provided the audio is strictly synchornous to the video it's possible to simplify things. Especially if you choose your audio sampling frequency to give exactly 2 sample per TV line. This is a bit low so you might choose 3 samples per TV line. Both are non-standard audio sampling rates. If you don't do this there will be different numbers of audio samples on each line (3 or 4 for 48kHz audio on 625, 2 or 3 for 44.1kHz audio on 625) so you'll need a FIFO at each end plus controls to keep the FIFO from over or under flowing.
Whatever you do, the audio must never contain 00 or FF as these are reserved for TRS codes. S272M has a unique and rather clunky implementation to stop this happening. All later ANC packet standards (audio on HD, timecode etc etc) are a lot cleaner and simpler.
Just a quick up date on what has been happening with this project.
I been trying to get Rf out of the FPGA. I have tried to get it via the R2R ladder but any attempt to use the ladder has resulted in severe pixellation, with 4 or 5 of the least significant bits out of the 9 in total having no effect. I think this is most lightly due to the 45 MHz carrier being above the maximum frequency of the ladder. I have looked on line to find the maximum frequency of R2R ladders but couldn't find any reference to it anywhere, not even in the data sheets of R2R ladder networks. I guess I could use a high speed DAC chip but I don't want to go down that route. The only way that I could get a watchable picture was to put a emitter follower on the output of the R2R ladder and connect the emitter of the emitter follower via a resistor to a pin of the FPGA. This pin of the FPGA is configured as open collector and is driven with the RF carrier. The result is modulated RF at this pin. A sketch of the basic circuit is attached below and also a photo of the results on screen. It is watchable but there is an underlying amount of noise which makes the picture somewhat murky. The noise is coming from the FPGA pin and can clearly be seen on a scope if the pin is connected to VCC via a resistor.
Cleaning up the noise would probably result in a good picture but I have decided to leave my attempts of getting RF from the FPGA at that and go down the MC44BS373 route. One of the reasons is that I could use the cheaper Cyclone II board which I am now using. When changing to the Cyclone II I took the opportunity to tidy up the files. I had a number of small files all connected through the top level file, resulting in the top level file being a pure rats nest of wiring. I merged a lot of the files together and connected the remaining files in a more streamlined fashion, this has made navigating the files easier.
Frank
I wanted to swap to the TVP5150 video decoder and try out some other components so I got a new PCB made up to accommodate them and the Cyclone II board that I am now using. Two photos of the partly populated PCB are attached below, none of the modulator components are fitted yet. The PCB measures 89 X 96 mm.
I have fitted a THS7314D video amp in place of the emitter follower that I was using after the R2R ladder. The THS7314D has 3 amplifiers in one package. Each amp has a fixed gain of X 2 and also a 8.5 MHz low pass filter. I will use one amp for video out and another to feed the modulator.
I am now oversampling the video output at twice pixel frequency, this together with the LPF in the video amp has cleaned up the output nicely.
Photos below of the video output, each photo is of the trailing edge of the sync pulse and the scope is set to 0.1 uS/div . The first is without oversampling. The second is with oversampling, the extra steps are clearly visible. The third is after it has passed through the video amp and LPF.
Frank
Good work Frank.
This has been a very interesting project, albeit a bit over my head being an analogue person.
I'm going to have a go at PC standards conversion, young Peter Scott will be here tomorrow and hopefully I'll get some tips and pointers.
Keep up the good work Frank.
Hi Trevor
I have been following the PC standards conversion tread with great interest. It looks very versatile. Peter has got great results from it. At some stage I would like to have a go but would need to check if any of the PC's I have have a suitable slot.
Frank
I had thought that in the 405 system that black level was always at the same level as blanking level, but in post 32 in
this thread over on UKVRRR there is a image from Wireless World 1956 and in it it states that black level has changed from 30% to 35% of peak white with blanking level remaining at 30%.
I have build into the converter a switchable pedestal which lifts black level from 300 mV above sync tip to 350 mV. When the pedestal is switched in the video is scaled to accommodate it.
I have tried it out on my Bush TV24 which dose not have fly back blanking and the pedestal dose help. It dose not eliminate the fly back lines but dose reduce there intensity.
Photos below is with no video present. First is of a trace without pedestal and the next is with the pedestal. The third is of the TV24 without pedestal and the last one is with pedestal.
Frank
From memory I think that the original 1936 spec had black level equal to blanking. I think some pedestal was introduced to help reduce fly back visibility and was then dropped. The final 405 spec certainly had no pedestal.
Hi Jeffrey
I guess it was dropped again as the benefits from it was limited.
I tried increasing black level to 400 mV above sync tips but the improvement was only marginally better than at 350 mV.
Frank
Pedestal also wastes transmitter power. Power that could be used to improve signal to noise ratio. Picture to sync ratio of 7 to 3 was chosen as a compromise between maintaining good sync and signal to noise on weak signals.
Hi Jeffrey
When you think of it in terms of transmitter power it is easy to understand why they dropped it particularly when the benefit got from it was limited.
Frank