Bug#897565: sox: null file (-n) contains inexplicable noise

[Bas Zoetekouw]

Package: sox
Version: 14.4.1-5+b2
Severity: normal

According to the sox manual, the --null option

>  [...] is equivalent to using a normal audio file that contains  an
>  infinite amount  of  silence,  and  as  such is not generally
>  useful unless used with an effect that specifies a finite time
>  length (such as trim or synth).

So, one would expect that a null input file converted to a different
format would result in total silence.  More explicitly,, one would
expect

> sox --null  -r16 -c1 foo.s8 trim 0: 0:0:1

to output 16 0-bytes.

However, instead of 0-bytes, we get wat seems to be 1-bit noise:

> ╰─▶ sox -n -r16 -c1 foo.s8 trim 0: 0:0:1 ; xxd foo.s8
> 00000000: 0000 0000 0000 00ff 0000 0000 0100 0000  ................
> 
> ╰─▶ sox -n -r16 -c1 foo.s8 trim 0: 0:0:1 ; xxd foo.s8
> 00000000: 0000 ff00 0001 0000 ff01 0000 0000 ff00  ................
> 
> ╰─▶ sox -n -r16 -c1 foo.s8 trim 0: 0:0:1 ; xxd foo.s8
> 00000000: ff00 0100 0001 0000 0000 0000 0000 0000  ................


Curiously enough, everything works as expected if we use _real_
silence as input:

> ╰─▶ sox -r16 -c1 -t s8 /dev/zero -r16 -c1 foo.s8 trim 0: 0:0:1 ; xxd foo.s8
> 00000000: 0000 0000 0000 0000 0000 0000 0000 0000  ................



-- System Information:
Debian Release: 9.4
  APT prefers stable
  APT policy: (620, 'stable'), (500, 'stable-updates'), (20, 'unstable'), (10, 'experimental')
Architecture: amd64 (x86_64)

Kernel: Linux 4.9.0-6-amd64 (SMP w/1 CPU core)
Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8), LANGUAGE=en_GB.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages sox depends on:
ii  libc6            2.24-11+deb9u3
ii  libgomp1         6.3.0-18+deb9u1
ii  libsox-fmt-alsa  14.4.1-5+b2
ii  libsox-fmt-base  14.4.1-5+b2
ii  libsox2          14.4.1-5+b2

sox recommends no packages.

Versions of packages sox suggests:
pn  libsox-fmt-all  <none>

-- no debconf information