[FFmpeg-devel] [PATCH] lavfi: add audio eval signal source

[Stefano Sabatini]

On date Thursday 2011-10-13 10:04:34 +0200, Nicolas George encoded:
> Le duodi 22 vendémiaire, an CCXX, Stefano Sabatini a écrit :
> > + at section aevalsrc
> 
> Cool idea, thanks.
> 
> > +Generate an audio signal generated by an expression.
> 
> The Department of Redundancy Department wants to thank you for having
> generated such an awesome introduction line.
> 
> > +This source accepts in input an expression, which is evaluated and
> > +used for generating a mono audio signal.
> 
> "to generate", I believe.
> 
> > + at table @option
> > + at item E, PI, PHI
> > +the corresponding mathematical approximated values for e
> > +(euler number), pi (greek PI), PHI (golden ratio)
> 
> Shouldn't it be made part of the eval system once and for all?
> 
> > + at item n
> > +the number of sample, starting from 0
> > +
> > + at item t
> > +time of the sample expressed in second, starting from 0
> 
> The place of "the" is moving. I fond the second one easier to understand.
> 

> > +Generate a sin signal with frequence 4400 Hz:
> > + at example
> > +aevalsrc="sin(4400*t)"
> 
> sin(4400*t*2*PI), or it will be 700 Hz.

Ouch!

> 
> > +    int64_t chlayouts[] = { AV_CH_LAYOUT_MONO, -1 };
> 
> As Michael said, the possibility to generate several channels would be cool.
> OTOH, it could be left to a separate amerge filter: (n channels, p channels)
> -> n+p channels.

Extended. I opted for the syntax:
expr1|expr2|...|exprN:opts

Assumptions: ":" and "|" must not appear in the expressions.

Right now the source generates planar audio, it shouldn't be hard to
add support for packed if people want that.
 
> > +    eval->var_values[VAR_N] = eval->n;
> > +
> > +    /* evaluate expression for each single sample */
> > +    for (i = 0; i < eval->nb_samples; i++) {
> > +        eval->var_values[VAR_T] = eval->var_values[VAR_N] * (double)1/eval->sample_rate;
> > +        *((double *) samplesref->data[0] + i) =
> > +            av_expr_eval(eval->expr, eval->var_values, NULL);
> > +        eval->var_values[VAR_N] = eval->n++;
> > +    }

> 
> The computation for VAR_N is wrong by one except on the first round because
> n++ is postincrementation, not preincrementation. I believe it could be more
> clearly written as:
> 
>     for (i = 0; i < eval->nb_samples; i++) {
>         eval->var_values[VAR_N] = eval->n;
>         eval->var_values[VAR_T] = eval->var_values[VAR_N] * (double)1/eval->sample_rate;
>         *((double *) samplesref->data[0] + i) =
>             av_expr_eval(eval->expr, eval->var_values, NULL);
>         eval->n++;
>     }
> 
> with possibly the eval->n++ part of the third for clause.

Fixed.

...

Complete example command:
$ ffplay -f lavfi "aevalsrc=abs(sin(1/10*2*PI*t))*sin(440*2*PI*t)|(1-abs(sin(1/10*t*2*PI)))*sin(450*2*PI*t + PI*sin(2*PI*t)), aconvert=s16:stereo:packed"

note that the aconvert=s16:stereo:packed is (unfortunately) required
or there will be a conversion to mono, as the auto-inserted aconvert
merges all the formats supported by aconvert with the output filter
formats, and this is done *before* to merge the formats supported by
aevalsrc and aconvert (but I'm thinking to add a new aconvert mode for
forcing aconvert to use the same input formats whenever it's
possible).
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