Editing MPEG-1 (section)

====Quality====
Subjective audio testing by experts, in the most critical conditions ever implemented, has shown MP2 to offer [[Transparency (data compression)|transparent]] audio compression at 256&nbsp;kbit/s for 16-bit 44.1&nbsp;kHz [[Red Book (audio CD standard)|CD audio]] using the earliest reference implementation (more recent encoders should presumably perform even better).<ref name=mpeg_faqs1>{{Citation |first1=Mark |last1=Adler |first2=Harald |last2=Popp |first3=Morten |last3=Hjerde |title=MPEG-FAQ: multimedia compression [1/9] |date=November 9, 1996 |publisher=faqs.org |url=http://www.faqs.org/faqs/mpeg-faq/part1/ |access-date=2016-11-11 |url-status=live |archive-url=https://web.archive.org/web/20170104010328/http://www.faqs.org/faqs/mpeg-faq/part1/ |archive-date=January 4, 2017 }}</ref><ref name=telos_audio/><ref name=audio_tutorial/><ref>C.Grewin, and T.Ryden, ''Subjective Assessments on Low Bit-rate Audio Codecs'', Proceedings of the 10th International AES Conference, pp 91 - 102, London 1991</ref> That (approximately) 1:6 compression ratio for CD audio is particularly impressive because it is quite close to the estimated upper limit of perceptual [[Entropy (information theory)|entropy]], at just over 1:8.<ref>J. Johnston, ''Estimation of Perceptual Entropy Using Noise Masking Criteria,'' in Proc. ICASSP-88, pp. 2524-2527, May 1988.</ref><ref>J. Johnston, ''Transform Coding of Audio Signals Using Perceptual Noise Criteria,'' IEEE Journal on Select Areas in Communications, vol. 6, no. 2, pp. 314-323, Feb. 1988.</ref> Achieving much higher compression is simply not possible without discarding some perceptible information.

MP2 remains a favoured lossy audio coding standard due to its particularly high audio coding performances on important audio material such as castanet, symphonic orchestra, male and female voices and particularly complex and high energy transients (impulses) like percussive sounds: triangle, glockenspiel and audience applause.<ref name=mpeg_faqs2/> More recent testing has shown that [[MPEG Multichannel]] (based on MP2), despite being compromised by an inferior matrixed mode (for the sake of backwards compatibility)<ref name=mpeg_faqs1/><ref name=audio_tutorial/> rates just slightly lower than much more recent audio codecs, such as [[Dolby Digital]] (AC-3) and [[Advanced Audio Coding]] (AAC) (mostly within the margin of error&mdash;and substantially superior in some cases, such as audience applause).<ref>Wustenhagen et al., ''Subjective Listening Test of Multi-channel Audio Codecs'', AES 105th Convention Paper 4813, San Francisco 1998</ref><ref name=ebu_surround_test_2007>{{Citation |last=B/MAE Project Group |title=EBU evaluations of multichannel audio codecs |date=September 2007 |publisher=[[European Broadcasting Union]] |url=http://www.ebu.ch/CMSimages/en/tec_doc_t3324-2007_tcm6-53801.pdf |archive-url=https://web.archive.org/web/20081030043259/http://www.ebu.ch/CMSimages/en/tec_doc_t3324-2007_tcm6-53801.pdf |url-status=dead |archive-date=2008-10-30 |access-date=2008-04-09 }}</ref> This is one reason that MP2 audio continues to be used extensively. The MPEG-2 AAC Stereo verification tests reached a vastly different conclusion, however, showing AAC to provide superior performance to MP2 at half the bitrate.<ref name=stereo_aac_tests>{{Citation |first1=David |last1=Meares |first2=Kaoru |last2=Watanabe |first3=Eric |last3=Scheirer |title=Report on the MPEG-2 AAC Stereo Verification Tests |pages=18 |date=February 1998 |publisher=[[International Organization for Standardization|ISO]]/[[International Electrotechnical Commission|IEC]] |url=http://sound.media.mit.edu/mpeg4/audio/public/w2006.pdf |access-date=2016-11-11 |url-status=dead |archive-url=https://web.archive.org/web/20080414072345/http://sound.media.mit.edu/mpeg4/audio/public/w2006.pdf |archive-date=April 14, 2008 }}</ref> The reason for this disparity with both earlier and later tests is not clear, but strangely, a sample of applause is notably absent from the latter test.

Layer II audio files typically use the extension ".mp2" or sometimes ".m2a".