[FFmpeg-user] Multiply audio samples

[Paul B Mahol]

On 9/12/18, Michael Koch <astroelectronic at t-online.de> wrote:
> Am 12.09.2018 um 09:55 schrieb Paul B Mahol:
>> Hi,
>>
>> On 9/12/18, Michael Koch <astroelectronic at t-online.de> wrote:
>>> Is it possible to multiply audio samples from two sources?
>>> I'd like to downconvert the 15kHz to 25kHz range to the 0 to 10kHz
>>> range. First apply a strong 15kHz highpass filter, then multiply the
>>> signal with a 15kHz sine wave (which produces sum and difference
>>> signals), then apply a 10kHz lowpass filter.
>> There is no yet amultiply audio filter, but I could write it.
>>
>> Am I correct that for it you need two audio streams as input and one
>> audio stream as output?
>
> yes, two inputs and one output. I think it would be best if it works
> independant of bit depth. First normalize the input signals to -1...+1
> range, then multiply them so that the result is also in the -1...+1
> range, then normalize back to 16bit or 24bit range.

It will internally work with floating-point numbers only (32bits or 64bits).
Normalization is done with other filters.

Meanwhile you can use sox multiply effect.

Note that sox internally operates with 32bit integers.