Microsoft Embedded Audio Coder (EAC)

Microsoft's Embedded Audio Codec (EAC) is an experimental audio coder offering fine-grain bitrate scalability. The coding algorithm is described in [1], which also compares the performance of EAC against an earlier version of the Scala codec.

The following single-channel sound files compare coding efficiencies of EAC and Scala codecs at two different bitrates. All files are approximately 6 seconds in length with 44.1 kHz sample rate; samples are presented both as compressed bitstreams and uncompressed (decoded) .wav files. For verification, the EAC bitstreams can be decoded using the EAC player, while Scala bitstreams can be decoded using the demonstration decoder software available from this site.

 

Signal	Original file (uncoded)		Mono Bitrate
			32 kbit/s		64 kbit/s
			EAC	SCALA	EAC	SCALA
harpsichord	harp.wav	bitstream	harp32.eac	harp32.sab	harp64.eac	harp64.sab
		decoded wav	harp32_eac.wav	harp32_sac.wav	harp64_eac.wav	harp64_sac.wav
female voice	voice.wav	bitstream	voice32.eac	voice32.sab	voice64.eac	voice64.sab
		decoded wav	voice32_eac.wav	voice32_sac.wav	voice64_eac.wav	voice64_sac.wav
castanets	cast.wav	bitstream	cast32.eac	cast32.sab	cast64.eac	cast64.sab
		decoded wav	cast32_eac.wav	cast32_sac.wav	cast64_eac.wav	cast64_sac.wav

 

While EAC performs well at lower bitrates for some signals including harpsichord, other signals such as female voice and castanets suffer from clearly audible coding artifacts at 32 kbit/s. At the higher rate of 64 kbit/s, our listening tests indicate lower average coding efficiency for EAC compared to the Scala codec. Reduced coding efficiency at higher bitrates is reflected in higher average transparency bitrates, measured for EAC at 110 kb/s (mono) and 168 kbit/s (stereo), compared to the Scala codec which achieves transparency at 80 kb/s (mono) and 132 kbit/s (stereo). Error-resilient coding is a further advantage of the Scala codec.

References

[1] J. Li, "Embedded Audio Coding (EAC) with Implicit Psychoacoustic Masking", ACM Multimedia 2002, pp. 592-601, Nice, France, Dec.1-6, 2002.