I put a new release of MAD on the FTP site. It's a day later than promised, but I wanted to incorporate some patches Nicolas sent me.

This version has many changes. The highlights are:

libmad changes:

* Incorporated Nicolas Pitre's ARM assembly and parameterized scaling changes.

* Incorporated Andre McCurdy's ARM assembly optimization (used only if --enable-aso is given to `configure' to enable architecture-specific optimizations.)

* Reduced FPM_INTEL assembly to two instructions.

* Fixed accuracy problems with certain FPM modes in synth.c.

* Improved the accuracy of FPM_APPROX.

* Improved the accuracy of SSO.

* Added experimental rules for generating a libmad.so shared library.

madplay changes:

* PCM output is now dithered for better audio quality.

* Added a resampling feature for unsupported output sampling frequencies.

* Improved the OSS output module by falling back on 8-bit format if 16-bit is not available, and by using native 16-bit endianness.

* Added a dual channel output selection option.

The ARM changes produced a favorable effect on the accuracy of the output from libmad on ARM processors; ARM output is no longer the same as Intel output and instead now matches the 64bit output, but with better performance:

default with SSO with ASO with SSO+ASO FPM_APPROX 6.800e-05/L 6.775e-05/L 6.431e-05/L 6.431e-05/L FPM_64BIT 5.580e-08/F 1.007e-05/L 5.652e-08/F 1.008e-05/L FPM_ARM 5.580e-08/F 1.007e-05/L 5.652e-08/F 1.008e-05/L FPM_INTEL 9.000e-08/F 1.008e-05/L

/F means full compliance, /L means limited accuracy, /N means not compliant.

Perhaps the most significant change to `madplay' is the addition of PCM output dithering. This is an alternative to ordinary rounding that improves the audio quality by reducing the negative effects of quantization noise. Dithering is commonly used in professional mastering when reducing studio-quality audio to 16 bits for pressing onto a CD. Since MAD produces PCM samples with >24 bits, dithering is a good idea.

The dithered output sounds less "grainy" than non-dithered, although this is easier to perceive with 8-bit output than with 16. Best of all, however, dithering effectively increases the precision of the output because it allows bits below the LSB to be heard.

There are many possible dithering strategies, but the chosen algorithm is fairly simple: it merely involves keeping the cumulative quantization error less than the significance of the LSB. The effect of this is for the LSB to modulate in proportional agreement with the bits below it.

The only significant drawback with dithering is that it hinders an analytic examination of the output, such as compliance testing. Therefore, it can be turned off with a -d option to `madplay'.

As always, the release can be found here:

ftp://ftp.mars.org/pub/mpeg/

Cheers, -rob