audioop module - request to add a note

[ncryptor]

BPO	1700821
Nosy	@birkenfeld, @rhettinger

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/birkenfeld'
closed_at = <Date 2008-03-24.09:34:51.841>
created_at = <Date 2007-04-14.23:07:14.000>
labels = ['docs']
title = 'audioop module - request to add a note'
updated_at = <Date 2008-03-24.09:34:51.826>
user = 'https://bugs.python.org/ncryptor'

bugs.python.org fields:

activity = <Date 2008-03-24.09:34:51.826>
actor = 'georg.brandl'
assignee = 'georg.brandl'
closed = True
closed_date = <Date 2008-03-24.09:34:51.841>
closer = 'georg.brandl'
components = ['Documentation']
creation = <Date 2007-04-14.23:07:14.000>
creator = 'ncryptor'
dependencies = []
files = []
hgrepos = []
issue_num = 1700821
keywords = []
message_count = 3.0
messages = ['31788', '31789', '64408']
nosy_count = 3.0
nosy_names = ['georg.brandl', 'rhettinger', 'ncryptor']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue1700821'
versions = []

[ncryptor]

The audioop function lin2lin converts sample sizes of linear pcm fragments.
However, when converting to 8-bit, there is a pitfall (that wasted for me a few hours).
In some audio formats, such as .WAV files, 16 and 32 bit samples are signed, but 8 bit samples are unsigned. So when converting from 2 to 1 byte wide samples, you need to also add 128 to the result.
Example code:

new_frames = audioop.lin2lin(frames, old_width, new_width)
if 1 == new_width:
   new_frames = audioop.bias(new_frames, 1, 128)

the same, in reverse, has to be applied when converting from 8 to 16 or 32 bit width samples.

This wasted so much time for me and is hardly documented.

[rhettinger]

I will add a note.

[birkenfeld]

Fixed in r61842.