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  1. Member Skiller's Avatar
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    Originally Posted by ConsumerDV View Post
    On the other hand maybe an ADC can switch from 13.5 MHz to 12.27 MHz and capture natively, I guess it depends on how it is clocked.
    Nobody in their right mind would design it that way. There is no reason to, and it is even blatantly wrong to sample below 13.5 MHz. 13.5 MHz is the minimum required to sample an analog standard def signal so that it can be faithfully recreated (again, Nyquist theorem). It is much less troublesome and way easier to implement to just apply digital scaling afterwards.
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  2. Originally Posted by Skiller View Post
    Originally Posted by ConsumerDV View Post
    On the other hand maybe an ADC can switch from 13.5 MHz to 12.27 MHz and capture natively, I guess it depends on how it is clocked.
    Nobody in their right mind would design it that way. There is no reason to, and it is even blatantly wrong to sample below 13.5 MHz. 13.5 MHz is the minimum required to sample an analog standard def signal so that it can be faithfully recreated (again, Nyquist theorem). It is much less troublesome and way easier to implement to just apply digital scaling afterwards.
    13.5 MHz is needed for interoperation between broadcast-quality 525/50 and 625/50, but if you want other resolution you can use another sampling frequency. Modern digital devices are programmable. If there is a suitable base clock from which other frequencies can be derived, why not?

    For example, for 29.97 fps:

    960×480i - 18.0 MHz
    864×480i - 16.38 MHz
    720×480i - 13.50 MHz
    704×480i - 13.50 MHz
    640×480i - 12.27 MHz
    544×480i - 10.12 MHz
    528×480i - 9.900 MHz
    480×480i - 9.900 MHz
    352×480i - 6.750 MHz
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  3. One cannot just reduce the sampling rate. One would also have to reduce the spectral bandwidth of the sampled analog video signal accordingly (low pass filtering to prevent aliasing), which adds some extra complexity. (By the way digital downscaling must also be preceeded by lowpass filtering, but it is easier done in the digital domain by software than in the analog domain.)
    For special applications there exist legacy low bandwidth video systems (low framerate / low resolution), driven by the non-availability of sufficient channel bandwidth (transmission or storage medium), or prohibitive cost of bandwidth for special applications. The trend goes the opposite direction, and standards - which are always a compromise - are mandatory for device and system interoperability and cost reduction.
    Last edited by Sharc; 1st Apr 2022 at 18:46.
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  4. Member Skiller's Avatar
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    Originally Posted by ConsumerDV View Post
    Modern digital devices are programmable. If there is a suitable base clock from which other frequencies can be derived, why not?
    Sure, changing the base sampling rate is not really the issue. Nyquist is. Have you got an idea what it means? It means you need to take at least twice the amount of samples of the bandwidth of the signal. To sample at a lower speed you would need to limit the bandwidth down subsequently. If you ignore this, as Sharc says, it will lead to Aliasing and/or Moiré.

    In other words, it also requires a programmable low-pass filter on the board to do this in the analog domain. Therefore I would go as far as to guarantee you won't find a capture device which does it this way. It's awkward, more expensive and not better than doing it in software.
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  5. Originally Posted by Sharc View Post
    One cannot just reduce the sampling rate. One would also have to reduce the spectral bandwidth of the sampled analog video signal accordingly (low pass filtering to prevent aliasing), which adds some extra complexity. (By the way digital downscaling must also be preceeded by lowpass filtering, but it is easier done in the digital domain by software than in the analog domain.)
    Thanks, Sharc! I appreciate the explanation.
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