18-11-2018, 07:03 AM
Video scaling is probably best done before or after the oversampler. Only needs a multiplier and adder. Also good idea to have a limiter so you can't accidentally go from 511 round to 0 or vice versa.
For audio decide on what digital range represents full modulation. Let's say it's 16 bit audio with a potential range of 0-65535. If we say that 32768 represents zero audio then 0 and 65535 are maximum possible excursions. Then using a multiplier and adder as required get the modulator to give the required output, 6dB below vision. Best to experiment by generating a full scale audio tone at carrier sample rate. Easily done with a DTO and sine table. Then you can easily see the modulated carrier on a scope.
The only purpose of the gain control before the ADC is to match the analogue audio amplitude to the ADC's input range. You really want AC coupling somewhere so that zero audio will map to 32768 regardless of the gain setting. I think you will find this easier if you use an ADC that's designed for audio, rather than the general purpose ADC you're currently using.
In practice you never want to utilise full 100% modualation as it will cause the detectors in receivers to distort. In practice you set up the gain control until it distorts and then back it off a bit.
For audio decide on what digital range represents full modulation. Let's say it's 16 bit audio with a potential range of 0-65535. If we say that 32768 represents zero audio then 0 and 65535 are maximum possible excursions. Then using a multiplier and adder as required get the modulator to give the required output, 6dB below vision. Best to experiment by generating a full scale audio tone at carrier sample rate. Easily done with a DTO and sine table. Then you can easily see the modulated carrier on a scope.
The only purpose of the gain control before the ADC is to match the analogue audio amplitude to the ADC's input range. You really want AC coupling somewhere so that zero audio will map to 32768 regardless of the gain setting. I think you will find this easier if you use an ADC that's designed for audio, rather than the general purpose ADC you're currently using.
In practice you never want to utilise full 100% modualation as it will cause the detectors in receivers to distort. In practice you set up the gain control until it distorts and then back it off a bit.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
