On Fri, Apr 14, 2023 at 3:45 PM Luke Koch <lu.ale.koch@xxxxxxxxx> wrote: > > > On Thu, Apr 13, 2023 at 3:50 PM Muni Sekhar <munisekharrms@xxxxxxxxx> wrote: > > > > Hi all, > > > > I recorded the silence on the default audio card by running "arecord > > -d 5 test.wav." > > > > Once it exited, I checked the contents by running "hexdump test.wav," > > and I see that silence is recorded as a sequence of 0x80 instead of > > 0x00. > > > > 0000000 4952 4646 0024 8000 4157 4556 6d66 2074 > > 0000010 0010 0000 0001 0001 1f40 0000 1f40 0000 > > 0000020 0001 0008 6164 6174 0000 8000 8080 8080 > > 0000030 8080 8080 8080 8080 8080 8080 8080 8080 > > 0000040 8080 8080 8080 8080 8080 8080 8080 8080 > > ... > > > > What might be the reason for silence being recorded as 0x80 instead of 0x0? > > > > > > -- > > Thanks, > > Sekhar > > This is what I found after Googling a bit: > Since you didn't specify a format, arecord defaults to 8 bit format (U8). > Being unsigned, a negative value for maximum negative amplitude is impossible. Therefore the value is given a bias of 128, > making 0 the maximum negative amplitude, 255 the maximum positive, and 128 the center point (or silence). Thanks for the clarification. So I tried passing signed 8-bit format (S8), and then the arecord died immediately, as mentioned below. $ arecord -f S8 test.wav Recording WAVE 'test.wav' : Signed 8 bit, Rate 8000 Hz, Mono arecord: begin_wave:2481: Wave doesn't support S8 format... For other format S16_LE, the arecord hangs for a few seconds and throws an I/O error. $ arecord -f S16_LE test.wav Recording WAVE 'test.wav' : Signed 16 bit Little Endian, Rate 8000 Hz, Mono arecord: pcm_read:2032: read error: Input/output error I am not sure why recording works only on the default format of U8. For other formats, the record either dies immediately or hangs for a timeout and then throws an I/O error. Any ideas about this behaviour? > > Best Regards, > Luke > > > -- Thanks, Sekhar
