Support Apple AIFF-C pseudo compression in aifc.py

[thruston]

BPO	30077
Nosy	@ronaldoussoren, @ned-deily, @ambv, @serhiy-storchaka, @zhangyangyu, @thruston, @dnknth
PRs	bpo-30077 Update aifc.py to support Apple AIFF-C "sowt" #1156 bpo-30077: Add aifc/sowt pseudo-compression #24449

^{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/serhiy-storchaka'
closed_at = <Date 2021-08-13.11:33:51.235>
created_at = <Date 2017-04-15.18:44:39.148>
labels = ['OS-mac', 'type-feature', 'library', '3.11']
title = 'Support Apple AIFF-C pseudo compression in aifc.py'
updated_at = <Date 2021-08-13.20:44:39.965>
user = 'https://github.com/thruston'

bugs.python.org fields:

activity = <Date 2021-08-13.20:44:39.965>
actor = 'thruston'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2021-08-13.11:33:51.235>
closer = 'lukasz.langa'
components = ['Library (Lib)', 'macOS']
creation = <Date 2017-04-15.18:44:39.148>
creator = 'thruston'
dependencies = []
files = []
hgrepos = []
issue_num = 30077
keywords = ['patch']
message_count = 8.0
messages = ['291727', '291761', '291762', '291766', '386497', '399529', '399530', '399558']
nosy_count = 7.0
nosy_names = ['ronaldoussoren', 'ned.deily', 'lukasz.langa', 'serhiy.storchaka', 'xiang.zhang', 'thruston', 'yussuf.ozkan']
pr_nums = ['1156', '24449']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue30077'
versions = ['Python 3.11']

[thruston]

aifc.py fails to open AIFF files containing the compression type "sowt" in the COMM chunk with an "unsupported compression type" error.

This compression type is an Apple specific extension that signals that the data is not actually compressed but is stored uncompressed in little Endian order.

Supporting it would require a trivial change to allow the compression type as a byte-string and to add a do-nothing _convert routine.

This would allow aifc.py to be used with AIFF files on Apple macOS.

[zhangyangyu]

Anyway, such a function addition could only get into 3.7.

[serhiy-storchaka]

Seems the "sowt" compression is not just a no-op operation. Perhaps it should change a byte order.

Needed tests and test files for the new compression.

[thruston]

I think it's probably better as a no-op. Currently in my patch I just put

    return data

You could do

return data[::-1]

to reverse the sample, so that it came out as big endian, but if you leave it as a no-op then the data is returned in the native format for Intel Mac OS, which is the whole point of the "sowt" encoding. Either way, the user can easily reverse the data in a unpack format if they want to, but in my own testing it seemed easier and simpler just to leave it in the native format. Then basically "it just works".

The patch as I've proposed it works fine on my MacOS boxes, but I have only tested it with AIFF-C files from one source.

[dnknth]

If you read a normal uncompressed AIFF with aifc, the samples are in big-endian order and need to be converted before they can be played on little-endian systems.

As all other compression schemes produce big-endian data, so should 'SOWT' encoded AIFF. Therefore, the bytes of each sample should be swapped on read and write.

See also: https://en.wikipedia.org/wiki/Audio_Interchange_File_Format#AIFF_on_Mac_OS_X

[ambv]

New changeset e4ed9d2 by dnknth in branch 'main':
bpo-30077: Add support for Apple aifc/sowt pseudo-compression (GH-24449)
e4ed9d2