Here are the graphs for the original test material used as the reference to compare all other data. Don't expect the flat frequency graph we are all used to seeing as the plot is derived from real music and not a signal generator sweep.

Reference CD Audio Frequency Response

Reference CD Audio Spectral View

As can be expected with most music the vast majority of spectral energy is concentrated at the lower frequencies. There is however significant frequency content at the higher frequencies and as you can see the frequency plot continues right up to 22kHz. Please take note of the fine detail in the spectral view even at high frequencies as it is here that you will be able to see considerable differences in the various codecs.

The bright pillars of red on the spectral graph are individual notes being played on the trumpet. As you can see the trumpet produces a lot of high intensity high frequency harmonics and is a reason why I chose this recording for the test.

This recording has two other noteworthy characteristics that I used when making comparative listening tests. The first are faint but clearly distinguishable noises made by the valves in the trumpet hitting their stops in certain parts of the recording. The second are the soft but clearly defined notes of the harpsichord in the accompaniment. I will comment on these features for the various codecs.

The next few pages have the graphs produced from MP3 output encoded by the ACM pro codec which now can produce output up to 256kb/s. The high quality setting was always used.

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