interpreting frequency plots
Reply #19 – 2013-03-18 17:43:58
Yes, at least they're fairly smooth. To be clear, I'm assuming you have a spreadsheet that contains columns Frequency (Hz), Left (dBFS), Right (dbFS) for all frequency bins of the power spectrum of each file. First, there's the original file from the first CD release, which you plotted in your first upload here . Then there's the file from the CD remaster that you've called 'AF'. Then there's the file from the HD release (downsampled from 96kHz stereo to 44.1kHz stereo to make it comparable to the CDs - your downsampling settings looked fine, by the way). As you want to compare ORIG, AF and HD, you then subtract the values of ORIG from the values in AF and the values in HD to obtain the difference in dB for each version compared to the original CD release, and it's these difference values that you're plotting against frequency. (Difference in dB in logarithmic domain is equivalent to dividing in the linear domain, hence I use the term 'normalize' below) In these difference plots you cannot identify the notes of the musical scale that are present because you've normalized against the original CD's frequency response to measure only the difference in EQ (and the difference in filtering at low and high end and thus you've normalized out any ripple that was originally present from tones that are notes on the musical scale (albeit averaged over the whole file). Note that many sounds that aren't on the musical scale will be present thanks to noiselike sounds (sibilant vocal sounds and hissing noises, drums and hi-hat hits and similar untuned percussion all have a fairly spread spectrum showing no peaks around the notes of the musical scale), so when averaging over the whole file, there's likely to be plenty of power in those parts of the spectrum to successfully normalize against. The original frequency response shows a 20 kHz low pass filter, which is recommended by the Red Book standard for audio CDs but isn't applied by Audition's downsampling filter or in many modern CD releases that often use more of a brickwall filter just below 22.05kHz. It also shows relatively little content below in the deep bass (open bass-E on a 4-string bass guitar has a fundamental frequency of about 41Hz). Your plots now show the EQ curve you'd have to apply to the original CD to obtain the same tonal balance as the AF or HD releases (and the negative of those dB values will turn AF or HD into the original tonal balance). The exception is in ranges where the original or the reissued versions have very low (essentially zero) audible content, which I guess means below about 40Hz and above about 16 kHz to 20 kHz, where you're comparing very small numbers as a ratio (I'm actually surprised the differences are so low - within 10 dB). If you wanted to match the EQ between various recordings, you'd probably get close by reading off values and plugging them into an EQ. It may be that certain aspects of stereo separation have changed slightly between mixes, causing some of the left-right differences shown in your plots, although these are fairly minor and barely audible (mostly < 1 dB).