mpg123 output audio format

Enumerations
enum	mpg123_channelcount { MPG123_MONO = 1 , MPG123_STEREO = 2 }

Functions
MPG123_EXPORT void	mpg123_rates (const long **list, size_t *number)
MPG123_EXPORT void	mpg123_encodings (const int **list, size_t *number)
MPG123_EXPORT int	mpg123_encsize (int encoding)
MPG123_EXPORT int	mpg123_format_none (mpg123_handle *mh)
MPG123_EXPORT int	mpg123_format_all (mpg123_handle *mh)
MPG123_EXPORT int	mpg123_format (mpg123_handle *mh, long rate, int channels, int encodings)
MPG123_EXPORT int	mpg123_format2 (mpg123_handle *mh, long rate, int channels, int encodings)
MPG123_EXPORT int	mpg123_format_support (mpg123_handle *mh, long rate, int encoding)
MPG123_EXPORT int	mpg123_getformat (mpg123_handle *mh, long *rate, int *channels, int *encoding)
MPG123_EXPORT int	mpg123_getformat2 (mpg123_handle *mh, long *rate, int *channels, int *encoding, int clear_flag)

Detailed Description

Functions to get and select the format of the decoded audio.

Before you dive in, please be warned that you might get confused by this. This seems to happen a lot, therefore I am trying to explain in advance. If you do feel confused and just want to decode your normal MPEG audio files that don't alter properties in the middle, just use mpg123_open_fixed() with a fixed encoding and channel count and forget about a matrix of audio formats. If you want to get funky, read ahead ...

The mpg123 library decides what output format to use when encountering the first frame in a stream, or actually any frame that is still valid but differs from the frames before in the prompted output format. At such a deciding point, an internal table of allowed encodings, sampling rates and channel setups is consulted. According to this table, an output format is chosen and the decoding engine set up accordingly (including optimized routines for different output formats). This might seem unusual but it just follows from the non-existence of "MPEG audio files" with defined overall properties. There are streams, streams are concatenations of (semi) independent frames. We store streams on disk and call them "MPEG audio files", but that does not change their nature as the decoder is concerned (the LAME/Xing header for gapless decoding makes things interesting again).

To get to the point: What you do with mpg123_format() and friends is to fill the internal table of allowed formats before it is used. That includes removing support for some formats or adding your forced sample rate (see MPG123_FORCE_RATE) that will be used with the crude internal resampler. Also keep in mind that the sample encoding is just a question of choice – the MPEG frames do only indicate their native sampling rate and channel count. If you want to decode to integer or float samples, 8 or 16 bit ... that is your decision. In a "clean" world, libmpg123 would always decode to 32 bit float and let you handle any sample conversion. But there are optimized routines that work faster by directly decoding to the desired encoding / accuracy. We prefer efficiency over conceptual tidyness.

People often start out thinking that mpg123_format() should change the actual decoding format on the fly. That is wrong. It only has effect on the next natural change of output format, when libmpg123 will consult its format table again. To make life easier, you might want to call mpg123_format_none() before any thing else and then just allow one desired encoding and a limited set of sample rates / channel choices that you actually intend to deal with. You can force libmpg123 to decode everything to 44100 KHz, stereo, 16 bit integer ... it will duplicate mono channels and even do resampling if needed (unless that feature is disabled in the build, same with some encodings). But I have to stress that the resampling of libmpg123 is very crude and doesn't even contain any kind of "proper" interpolation.

In any case, watch out for MPG123_NEW_FORMAT as return message from decoding routines and call mpg123_getformat() to get the currently active output format.

Enumeration Type Documentation

mpg123_channelcount

enum mpg123_channelcount

They can be combined into one number (3) to indicate mono and stereo...

Enumerator
MPG123_MONO	mono
MPG123_STEREO	stereo

Definition at line 608 of file mpg123.h.

Function Documentation

mpg123_rates()

MPG123_EXPORT void mpg123_rates	(	const long **	list,
		size_t *	number
	)

An array of supported standard sample rates These are possible native sample rates of MPEG audio files. You can still force mpg123 to resample to a different one, but by default you will only get audio in one of these samplings. This list is in ascending order.

Parameters

list	Store a pointer to the sample rates array there.
number	Store the number of sample rates there.

mpg123_encodings()

MPG123_EXPORT void mpg123_encodings	(	const int **	list,
		size_t *	number
	)

An array of supported audio encodings. An audio encoding is one of the fully qualified members of mpg123_enc_enum (MPG123_ENC_SIGNED_16, not MPG123_SIGNED).

Parameters

list	Store a pointer to the encodings array there.
number	Store the number of encodings there.

mpg123_encsize()

MPG123_EXPORT int mpg123_encsize

(

int

encoding

)

Return the size (in bytes) of one mono sample of the named encoding.

Parameters

encoding

The encoding value to analyze.

Returns

positive size of encoding in bytes, 0 on invalid encoding.

mpg123_format_none()

MPG123_EXPORT int mpg123_format_none

(

mpg123_handle *

mh

)

Configure a mpg123 handle to accept no output format at all, use before specifying supported formats with mpg123_format

Parameters

mh

handle

Returns

MPG123_OK on success

mpg123_format_all()

MPG123_EXPORT int mpg123_format_all

(

mpg123_handle *

mh

)

Configure mpg123 handle to accept all formats (also any custom rate you may set) – this is default.

Parameters

mh

handle

Returns

MPG123_OK on success

mpg123_format()

MPG123_EXPORT int mpg123_format	(	mpg123_handle *	mh,
		long	rate,
		int	channels,
		int	encodings
	)

Set the audio format support of a mpg123_handle in detail:

Parameters

mh	handle
rate	The sample rate value (in Hertz).
channels	A combination of MPG123_STEREO and MPG123_MONO.
encodings	A combination of accepted encodings for rate and channels, p.ex MPG123_ENC_SIGNED16 | MPG123_ENC_ULAW_8 (or 0 for no support). Please note that some encodings may not be supported in the library build and thus will be ignored here.

Returns

MPG123_OK on success, MPG123_ERR if there was an error.

mpg123_format2()

MPG123_EXPORT int mpg123_format2	(	mpg123_handle *	mh,
		long	rate,
		int	channels,
		int	encodings
	)

Set the audio format support of a mpg123_handle in detail:

Parameters

mh	handle
rate	The sample rate value (in Hertz). Special value 0 means all rates (the reason for this variant of mpg123_format()).
channels	A combination of MPG123_STEREO and MPG123_MONO.
encodings	A combination of accepted encodings for rate and channels, p.ex MPG123_ENC_SIGNED16 | MPG123_ENC_ULAW_8 (or 0 for no support). Please note that some encodings may not be supported in the library build and thus will be ignored here.

Returns

MPG123_OK on success, MPG123_ERR if there was an error.

mpg123_format_support()

MPG123_EXPORT int mpg123_format_support	(	mpg123_handle *	mh,
		long	rate,
		int	encoding
	)

Check to see if a specific format at a specific rate is supported by mpg123_handle.

Parameters

mh	handle
rate	sampling rate
encoding	encoding

Returns

0 for no support (that includes invalid parameters), MPG123_STEREO, MPG123_MONO or MPG123_STEREO|MPG123_MONO.

mpg123_getformat()

MPG123_EXPORT int mpg123_getformat	(	mpg123_handle *	mh,
		long *	rate,
		int *	channels,
		int *	encoding
	)

Get the current output format written to the addresses given. If the stream is freshly loaded, this will try to parse enough of it to give you the format to come. This clears the flag that would otherwise make the first decoding call return MPG123_NEW_FORMAT.

Parameters

mh	handle
rate	sampling rate return address
channels	channel count return address
encoding	encoding return address

Returns

MPG123_OK on success

mpg123_getformat2()

MPG123_EXPORT int mpg123_getformat2	(	mpg123_handle *	mh,
		long *	rate,
		int *	channels,
		int *	encoding,
		int	clear_flag
	)

Get the current output format written to the addresses given. This differs from plain mpg123_getformat() in that you can choose not to clear the flag that would trigger the next decoding call to return MPG123_NEW_FORMAT in case of a new format arriving.

Parameters

mh	handle
rate	sampling rate return address
channels	channel count return address
encoding	encoding return address
clear_flag	if true, clear internal format flag

Returns

MPG123_OK on success