Speex FAQ - Revision history

MrZeus: /* Can Speex run on fixed-point processors or DSPs? */

2017-11-09T15:02:27Z

Can Speex run on fixed-point processors or DSPs?

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	=== Can Speex run on fixed-point processors or DSPs? ===		=== Can Speex run on fixed-point processors or DSPs? ===

	Yes. You can compile Speex for fixed-point CPUs by passing the --enable-fixed-point option to the configure script or defining FIXED_POINT.		Yes. You can compile Speex for fixed-point CPUs by passing the '''--enable-fixed-point''' option to the configure script or defining '''FIXED_POINT'''.

	=== What architectures are supported? ===		=== What architectures are supported? ===

MrZeus: /* Why is Speex so slow on my iPaq (or insert any platform without an FPU)? */

2017-11-09T15:00:49Z

Why is Speex so slow on my iPaq (or insert any platform without an FPU)?

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	=== Why is Speex so slow on my iPaq (or insert any platform without an FPU)? ===		=== Why is Speex so slow on my iPaq (or insert any platform without an FPU)? ===

	You probably didn't build Speex with the ~~fixed-point option (~~--enable-fixed-point). Even if you did, not all modes have been ported to use fixed-point arithmetic, so the code may be slowed down by ~~a few~~ float operations left (e.g. in the ~~wideband~~ mode).		You probably didn't build Speex with the '''--enable-fixed-point''' option.

			Even if you did, not all modes have been ported to use fixed-point arithmetic, so the code may be slowed down by any float operations left (e.g. in the Wideband mode).

	=== I'm getting unusual background noise (hiss) when using libspeex in my application. How do I fix that? ===		=== I'm getting unusual background noise (hiss) when using libspeex in my application. How do I fix that? ===

MrZeus: /* Can Speex pass V.90/V.92 modem signals correctly? */ fix wiki link

2017-11-08T12:57:05Z

Can Speex pass V.90/V.92 modem signals correctly?: fix wiki link

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	I guess it all depends on the bit-rate used. Though no formal testing has yet been performed, I'd say it works correctly at 8 kbps and above. Make sure you don't use the lowest complexity (see SPEEX_SET_COMPLEXITY or --comp option), as it causes significant noise.		I guess it all depends on the bit-rate used. Though no formal testing has yet been performed, I'd say it works correctly at 8 kbps and above. Make sure you don't use the lowest complexity (see SPEEX_SET_COMPLEXITY or --comp option), as it causes significant noise.

	=== Can Speex pass [https://en.wikipedia.org/wiki/~~V.90_(recommendation)~~ V.90]/[https://en.wikipedia.org/wiki/V.92 V.92] modem signals correctly? ===		=== Can Speex pass [https://en.wikipedia.org/wiki/List_of_ITU-T_V-series_recommendations#Simultaneous_transmission_of_data_and_other_signals V.90] / [https://en.wikipedia.org/wiki/V.92 V.92] modem signals correctly? ===
			No it cannot, as that would break fundamental laws of information theory. If it '''could''' do that, I'd be '''very''' rich by now :-)
	If it could do that I'd be very rich by now :-) ~~No it cannot, as that would break fundamental laws of information theory.~~

	=== Does Speex have anything to do with the University of Sherbrooke? ===		=== Does Speex have anything to do with the University of Sherbrooke? ===

MrZeus: /* Why is encoding so slow compared to decoding? */

2017-06-08T16:00:22Z

Why is encoding so slow compared to decoding?

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	In the case of Speex, encoding consists of finding, for each vector of 5 to 10 samples, the entry that matches the best within a codebook consisting of 16 to 256 entries.		In the case of Speex, encoding consists of finding, for each vector of 5 to 10 samples, the entry that matches the best within a codebook consisting of 16 to 256 entries.
	When decoding, all that needs to be done is look up the right entry in the codebook using the encoded index. Since a lookup is much faster than a search, the decoder works much faster than the encoder.		When decoding, all that needs to be done is look up the right entry in the codebook using the encoded index. Since a lookup is much faster than a search, the decoder works much faster than the encoder.

	~~This behaviour ties in with the assumption that in most use cases, people will be encoding a file once, but then decode it multiple times.~~

	=== Why is Speex so slow on my iPaq (or insert any platform without an FPU)? ===		=== Why is Speex so slow on my iPaq (or insert any platform without an FPU)? ===

MrZeus: /* Where can I get information about how Speex works? */

2017-06-08T15:08:18Z

Where can I get information about how Speex works?

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	=== Where can I get information about how Speex works? ===		=== Where can I get information about how Speex works? ===

	There is not (yet) a complete description of the algorithm (except for the source code itself). However, several aspects are documented either in the [http://speex.org/docs/ manual] or in the paper [http://people.xiph.org/~jm/papers/aes120_speex_vorbis.pdf Improved Noise Weighting in CELP Coding of Speech — Applying the Vorbis Psychoacoustic Model To Speex].		There is not (yet) a complete description of the algorithm (except for the source code itself).

			However, several aspects are documented either in the [http://speex.org/docs/ manual] or in the paper [http://people.xiph.org/~jm/papers/aes120_speex_vorbis.pdf Improved Noise Weighting in CELP Coding of Speech — Applying the Vorbis Psychoacoustic Model To Speex].

	[[Category:Speex]]		[[Category:Speex]]

MrZeus: /* CELP, ACELP, what's the difference? */

2017-06-08T15:07:24Z

CELP, ACELP, what's the difference?

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	== Technical ==		== Technical ==

	=== ~~CELP, ACELP, what~~'s the difference? ===		=== What's the difference between CELP and ACELP? ===

	CELP stands for "Code Excited Linear Prediction", while ACELP stands for "Algebraic Code Excited Linear Prediction". That means ACELP is a CELP technique that uses an algebraic codebook represented as a sum of unit pulses, thus making the codebook search much more efficient. This technique was invented at the University of Sherbrooke and is now one of the most widely used ~~form~~ of CELP. Unfortunately, since it is patented, it could not be used in Speex.		CELP stands for "Code Excited Linear Prediction", while ACELP stands for "Algebraic Code Excited Linear Prediction".

			That means ACELP is a CELP technique that uses an algebraic codebook represented as a sum of unit pulses, thus making the codebook search much more efficient. This technique was invented at the University of Sherbrooke and is now one of the most widely used forms of CELP.

			Unfortunately, since ACELP is patented, it could not be used in Speex.

	=== Where can I get information about how Speex works? ===		=== Where can I get information about how Speex works? ===

MrZeus: /* Can I use Speex for 22.05 kHz, 44.1 kHz or 48 kHz signals? */

2017-06-08T14:59:55Z

Can I use Speex for 22.05 kHz, 44.1 kHz or 48 kHz signals?

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	=== Can I use Speex for 22.05 kHz, 44.1 kHz or 48 kHz signals? ===		=== Can I use Speex for 22.05 kHz, 44.1 kHz or 48 kHz signals? ===

	Yes, but you must resample the signal to a supported sample rate before encoding. Speex is optimised for 8 kHz and 16 kHz and it can (your mileage may vary) ~~encode 32 kHz files as well~~. Anything else is unsupported and tends to be heavily sub-optimal. You might as well use Vorbis instead. Note that Speex includes a resampler module as of version 1.2beta2. Refer to the section called Resampler in the Speex documentation.		Yes, but you must resample the signal to a supported sample rate before encoding.

			Speex is optimised for '''8 kHz''' and '''16 kHz''' and it can also encode 32 kHz files as well (your mileage may vary). Anything else is ''unsupported'' and tends to be ''heavily sub-optimal''. You might as well use Vorbis instead.

			Note that Speex includes a resampler module as of version '''1.2beta2'''. Refer to the section called Resampler in the Speex documentation.

	=== Should I use version 1.0.x or 1.2? ===		=== Should I use version 1.0.x or 1.2? ===

MrZeus: /* Why is encoding so slow compared to decoding? */

2017-06-08T14:56:05Z

Why is encoding so slow compared to decoding?

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	=== Why is encoding so slow compared to decoding? ===		=== Why is encoding so slow compared to decoding? ===

	For most kinds of compression, encoding is inherently slower than decoding. In the case of Speex, encoding consists of finding, for each vector of 5 to 10 samples, the entry that matches the best within a codebook consisting of 16 to 256 entries. ~~On the other hand~~, ~~at decoding~~ all that needs to be done is look up the right entry in the codebook using the encoded index. Since a lookup is much faster than a search, the decoder works much faster than the encoder.		For most kinds of compression, encoding is inherently slower than decoding.

			In the case of Speex, encoding consists of finding, for each vector of 5 to 10 samples, the entry that matches the best within a codebook consisting of 16 to 256 entries.
			When decoding, all that needs to be done is look up the right entry in the codebook using the encoded index. Since a lookup is much faster than a search, the decoder works much faster than the encoder.

			This behaviour ties in with the assumption that in most use cases, people will be encoding a file once, but then decode it multiple times.

	=== Why is Speex so slow on my iPaq (or insert any platform without an FPU)? ===		=== Why is Speex so slow on my iPaq (or insert any platform without an FPU)? ===

MrZeus: /* Can Speex pass V.9x modem signals correctly? */

2017-06-08T14:34:48Z

Can Speex pass V.9x modem signals correctly?

← Older revision		Revision as of 14:34, 8 June 2017
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	I guess it all depends on the bit-rate used. Though no formal testing has yet been performed, I'd say it works correctly at 8 kbps and above. Make sure you don't use the lowest complexity (see SPEEX_SET_COMPLEXITY or --comp option), as it causes significant noise.		I guess it all depends on the bit-rate used. Though no formal testing has yet been performed, I'd say it works correctly at 8 kbps and above. Make sure you don't use the lowest complexity (see SPEEX_SET_COMPLEXITY or --comp option), as it causes significant noise.

	=== Can Speex pass V.9x modem signals correctly? ===		=== Can Speex pass [https://en.wikipedia.org/wiki/V.90_(recommendation) V.90]/[https://en.wikipedia.org/wiki/V.92 V.92] modem signals correctly? ===

	If I could do that I'd be very rich by now :-) No it cannot, as that would break fundamental laws of information theory.		If it could do that I'd be very rich by now :-) No it cannot, as that would break fundamental laws of information theory.

	=== Does Speex have anything to do with the University of Sherbrooke? ===		=== Does Speex have anything to do with the University of Sherbrooke? ===

MrZeus: /* I converted some MP3s to Speex and the quality is bad. What's wrong? */

2017-06-08T14:08:55Z

I converted some MP3s to Speex and the quality is bad. What's wrong?

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	=== I converted some MP3s to Speex and the quality is bad. What's wrong? ===		=== I converted some MP3s to Speex and the quality is bad. What's wrong? ===

	This is called transcoding and it will always result in much poorer quality than the original MP3.		This is called '''[https://en.wikipedia.org/wiki/Transcoding transcoding]''' (converting from one lossy format to another) and it will always result in much poorer quality than the original MP3.

	~~Unless~~ you have a really good reason (smaller file size) ~~to do so, never transcode speech~~. ~~This is even valid for~~ self transcoding ~~(tandeming~~)~~: i~~.e. If you decode a Speex file and re-encode it again at the same bit-rate, you will lose quality.		Avoid transcoding speech, unless you have a ''really'' good reason to do so (e.g. you need a smaller file size).
			You should also avoid '''tandeming''' (self-transcoding). I.e. if you decode a Speex file and re-encode it again at the same bit-rate, you ''will'' lose quality.

	=== How does Speex compare to other proprietary codecs? ===		=== How does Speex compare to other proprietary codecs? ===