[FFmpeg-user] Differences between ffmpeg and ffplay

[Moritz Barsnick]

On Tue, Oct 02, 2018 at 23:33:04 +0200, Michael Koch wrote:
> As the next step, I want to hear the output live on the speakers. I did 
> use exactly the same command line as above, but I replaced ffmpeg by 
> ffplay and I omitted the output file. At the end of the console output 
> is an error message. Can't ffplay handle two inputs? Why doesn't ffplay 
> accept the command line that works fine with ffmpeg?

Indeed, ffplay can't handle multiple inputs that way.

> C:\Users\mkoch\Desktop\Ultrasonic Live>c://ffmpeg/ffplay -i test_15kHz.mp3 -f lavfi -i aevalsrc="sin(12000*2*PI*t):c=stereo:s=44100" -filter_complex "[0]volume=1,highpass=f=12000,highpass=f=12000,highpass=f=12000,highpass=f=12000[sound];[sound][1]amultiply,lowpass=f=10000,lowpass=f=10000,lowpass=f=10000,lowpass=f=10000"

But what you can do is to put both your lavfi source as well as the
input file into a filter chain. ffmpeg provides the "movie" and
"amovie" filter sources for this.

So you can probably do (I'm doing this as a wild guess, without
testing):

$ ffplay -f lavfi "amovie=test_15kHz.mp3[a0]; aevalsrc="sin(12000*2*PI*t):c=stereo:s=44100[a1]; [a0]volume=1,highpass=f=12000,highpass=f=12000,highpass=f=12000,highpass=f=12000[sound];[sound][a1]amultiply,lowpass=f=10000,lowpass=f=10000,lowpass=f=10000,lowpass=f=10000"

(I'm not sure whether you need the "graph" option, or what it's even
good for.)
The "lavfi" input device documentation has some examples.

Moritz