Spectral Comparisons

7/27/2019 Spectral Comparisons

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1. Introduction: lossless and lossy formats

This is not a guide. This section is here in order to visually show you thedifference between the different kinds of files treated in the EAC guides. It endswith a brief explanation of what transcoding is and why transcoding normally isa bad thing to do.

Our point of departure is the wave file: the .wav file that EAC produces as yourip an audio CD track. To be sure, .wav is really a container format that can holddifferent kinds of audio; in this case the content of the .wav file is "really" PCMaudio.

If you have ripped the CD properly, in accordance with the guides, the .wav filecontains exactly the same audio data as the original CD. Wave files are ratherlarge, though, which makes them awkward for transfer and/or storage. This iswhere FLAC enters the picture. It stands for "Free Lossless Audio Codec". Itcompresses the audio data without removing any of it, just like Zip and RARcompress data without removing any of it. This is why FLAC is properly called alossless format. With the right kind of cue sheet, you can re-create a copy ofyour CD from your archived FLAC files any time.

Lossy formats can save space by removing some audio data, for example bycutting down on the number of kilobits used per second of audio and/orremoving "unnecessary" audio data according to some algorithm, in addition tocompressing the files. This way the files can be made much smaller, but there isalways a trade-off between audio quality and file size.

A common standard for "acceptable" audio quality for mp3s is 128 kbps. It is aconstant bitrate (CBR) setting that you see, for example, with most iTunesdownloads (though they make use of the FhG encoder, not the superior LAMEencoder for mp3). Ogg Vorbis has been developed to focus on quality measuresrather than bitrates, but a rough equivalent to mp3 at 128 kbps would be the q3setting.

Others claim that the threshold for "transparency" (where you can't tell thedifference from the original) would be at 192 kbps for mp3s, and q6 (or evenq5) for Ogg Vorbis. For LAME mp3, there is also the variable bitrate (VBR) preset

V2 (target bitrate ~190) that would be the "transparent" VBR setting. (Variablebitrate, used in both the Ogg Vorbis -q presets and the LAME mp3 -V presets,means that the encoder analyses the audio data and only uses as high bitrate asis needed, i. e. lower bitrate for less complex (or silent) parts of the audio data.)

The pictures below are intended to show you the differences between differentformats and bitrate settings, from "best possible" to "transparent" to"acceptable".

2. Spectral view

The pictures are screenshots taken while using the "spectral view" in AdobeAudition (v. 1.5). The spectral view is a way of making some audio propertiesvisible. The screenshots show you the two stereo channels with the left channelon top. The horisontal axis is for time, the vertical axis shows the frequency ofthe audio data for each channel. The colours indicate the amplitude: from darkblue for the lowest decibel (softest) sounds to bright yellow for the highestdecibel (loudest) sounds.

These screenshots show a maximum of 22 kHz for each channel. That isbecause we are looking at CD rips with the standard frequency of 44.1 kHz(combined for both channels). Other recordings, such as some soundboard tapesor vinyl rips, may go much higher.

All the screenshots, except two of the FLAC ones, are for the same track: "I FeelYou" from Depeche Mode's Songs of Faith and Devotion. That is a reasonablycomplex audio track that serves to illustrate the differences between the differentencodings. The LAME mp3 and Ogg Vorbis files were created from the FLAC fileusing dBpoweramp.

3. FLAC

First out is the original FLAC file - if it were decompressed to WAV (PCM) itwould still look exactly the same. (In fact, any file you open in Audition is"converted" to WAV in order to show the waveform.) As you can see, it goes allthe way up to 22 kHz. The file size is 31.2 MB; the decompressed .wav filewould be 46.3 MB.


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FLAC (I Feel You, Depeche Mode) 31.2 MB

The following two pictures are here in order to illustrate that CD rips will look

different depending on the source and the kind of music. The first one is arecording originally made in 1937, digitally remastered from the analogue masterin 2000. It's the small jazz group Benny Goodman Quartet (clarinet, piano,vibraphone and drums) playing "Tea For Two". This is what old and most newacoustic recordings will look like. In other words, there is not necessarilyanything wrong with a CD rip that does not show audio data all the way up to22 kHz per channel. The second picture drives this fact home: it is MitsukoUchida playing the rondo from Mozart's KV. 545, recorded digitally in 1984 byPhilips' finest.

FLAC (small group jazz, 12.9 MB)


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FLAC (classical solo piano, 6.6 MB)

4. LAME mp3

LAME mp3, starting with the highest CBR setting, 320 kbps. The frequency hasbeen cut off at 20.5 kHz - that is one reason for the comparatively small size.

Another reason is the removal of some "unnecessary" audio data, that you cansee in the stripey character of these pictures as compared to the original FLAC.The V0 that follows should be equal in sound quality to the 320 kbps CBR, butit is smaller in size because lower bitrates have been used for less complex partsof the audio, and the frequency maximum is at 19.5 kHz. Also, in the V0 youcan see a "shelf" cut-off at 16 kHz, that is more pronounced in the "transparent"

V2 (max frequency at 18.5 kHz), and prominent as a clear cut-off for the"acceptable" 128 kbps CBR.

LAME mp3 320 kbps (I Feel You, Depeche Mode) 10.8 MB

LAME mp3 V0 (I Feel You, Depeche Mode) 8.8 MB


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LAME mp3 V2 (I Feel You, Depeche Mode) 6.6 MB

LAME mp3 128 kbps (I Feel You, Depeche Mode) 4.3 MB

5. Ogg Vorbis

Ogg Vorbis can go up to 500 kbps, and it thus has a higher "best possible"quality than mp3, illustrated here by the q10 setting. Ogg Vorbis uses a different

algorithm from mp3, and doesn't cut off the frequency for the higher qualitysettings: both the q10 and the "transparent" q6 shown here have more or lessaudio data preserved at 22 kHz, though much less for the q6. The "acceptable"quality q4, still reaches roughly 19 kHz, but you can clearly see that some audiodata is missing, compared to the original FLAC.


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Ogg Vorbis q10 (I Feel You, Depeche Mode) 15.4 MB



6. Transcodes

So, encoding to lossy formats gives you the advantage of small file size at theprice of losing some audio data. This simple fact should be enough to explainwhy you can not encode such files again, and get higher quality audio files. Yousimply can not regain lost audio data, not by increasing the bitrate, and not byconverting to another format.

The quality will actually decline in lossy to lossy encoding, even if you try to"increase" the quality by encoding to a higher bitrate, since the algorithm forremoving "unnecessary" audio data is applied once again. If you encode fromlossy to lossless, the quality will stay exactly the same, only the file size getsbigger. See the screenshots below. Yes, they look very much like the mp3 128and the V0 above, respectively, but these really are screenshots of transcodes.

Do not ever do it.


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Do not ever do it.

128 kbps LAME mp3 transcoded to 320 kbps LAME mp3 (I Feel You, Depeche

Mode) 10.8 MB

V0 LAME mp3 transcoded to FLAC (I Feel You, Depeche Mode) 30.4 MB

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Spectral Comparisons