The giveaway for low-mid nitrate MP3 is the high hats. The lower the nitrate, the more you get a sort of temporal ghosting that sounds like an almost “crunchy” swishy sizzle sort of sound, a bit like a jazz player using brushes, but more lo fi.
I agree, and my hypothesis is that it's exacerbated by the combination of three particular things:
1. It's a high frequency complex waveform with a fast envelope, so it demands bitrate.
2. Drum miking often involves multiple mics spaced apart, so more than one typically picks up any given cymbal with a phase offset, and those mics are panned quite differently, leading to a very "wide" result, i.e., left and right output is fairly uncorrelated as seen on a vectorscope [0].
3. A perceptual codec at a given total bitrate often sounds better when stored as a mid-side transformation (instead of storing a left channel and a right channel, store a L+R "mid" a.k.a. sum channel and a L-R "side" a.k.a. difference channel), also known as "joint stereo" which is a common flag on MP3 encoders, because it allows for assigning more bits to the mid channel (correlated signals) and fewer bits to the side channel (uncorrelated signals). More bits for mono center-panned stuff like vocals is the goal, which is generally for the best, but fewer bits remain available for wide stuff like those cymbals! Contrast with regular stereo mode where half of the total bitrate is assigned to each channel. MP3 below 256kbps typically needs joint stereo mode enabled in order to sound decent.
A realistic take on the issues invovled. I never knew what “joint stereo” meant. Great explanation.
If anyone has a good cymbal crash sample at 24/96 or better that they can provide, it seems like it would be a great example for intentional differentiation of various compressed versions.
FM radio as well: originally one channel, then a second was added, and the second is used for the difference signal (the first already being the sum signal, as with any conversion to mono). Overhauling the whole thing to broadcast left and right discretely would destroy backwards compatibility, and the newly-added subcarrier had worse SNR (thus receivers ignore it until reception is sufficiently strong) so it only made sense to use it for the difference.
