High shelf filters and lossy/lossless tests

The better codecs (AAC, LAME) don't alter the frequency response unless the bitrate is very low.

Quote:

Originally Posted by scuttle 

I've been intrigued by seeing several tests where lossy recordings, often at moderate bitrates, do better than lossless versions in testing, and I wonder if I've stumbled on an explanation:

 

Neither audio transducers nor power amplifiers are free of distortion, and distortion tends to increase rapidly at the lowest and highest frequencies. If the same transducer reproduces ultrasonics along with audible content, any nonlinearity will shift some of the ultrasonic content down into the audible range as an uncontrolled spray of intermodulation distortion products covering the entire audible spectrum. Nonlinearity in a power amplifier will produce the same effect. The effect is very slight, but listening tests have confirmed that both effects can be audible.

 

My thought is that lossy recordings are removing high frequency information that isn't very important - and may well be over many testers hearing range even if not literally ultrasonic - and which when present causes distortions either in output devices (ie speakers, iems, etc) or even the ear itself. 

 

 

What you've stumbled on isn't an explanation, it's an opinion without substantive evidence.  

The statement in bold is very black/white, and makes no allowance for the possibility that power amps can be highly linear, and pass the tiny ultrasonics found in some high-rate files without causing IMD, and that most transducers will just fail to respond to signals outside their range.  

The big question is: what listening tests have confirmed that both effects can be audible?  

The smaller question is: if the can be, are they typically, or is it a case of the extremes floating to the top?

If they cite the tests, we might stand a chance of evaluating the statement, otherwise its just opinion.  

CDs or their lossless versions don't contain much above 20 kHz either. Why should e.g. an mp3 do better if it rolls off a bit earlier?

Oh I see what you mean. 15 kHz seems a bit extreme though. You'd have to have high frequency hearing loss to not notice it in an ABX test.

I wouldn't compare a "smooth" roll off to a brickwall filter (sudden cut off).

Let me take the idea to the extreme: bandlimiting the audio to a single frequency, for example 1000 Hz, would eliminate intermodulation distortion.

Quote:

Originally Posted by jaddie 

What you've stumbled on isn't an explanation, it's an opinion without substantive evidence.  

 

The statement in bold is very black/white, and makes no allowance for the possibility that power amps can be highly linear, and pass the tiny ultrasonics found in some high-rate files without causing IMD, and that most transducers will just fail to respond to signals outside their range. 

Sure, a tweeter won't reproduce bass frequencies and a bass woofer won't reproduce treble but feeding the driver frequencies that are clearly out of its comfort zone is a bad idea. Either sound quality will be adversely affected or the driver will be damaged. I think that is fact.

Are tweeters in hi-fi speakers made to reproduce frequencies as high as 40 or even 90 kHz?

It seems the lower the bitrate, the higher the peak loudness for whatever that's worth.

Quote:

Originally Posted by scuttle 

 

How low is "very low"? It is somewhat lower br recordings I'm interested in - eg the recent 192 vs 320 test.

128 and below I believe. At 192 and above, the response should be stone flat.

It seems to be hard to get concrete information on this. I've seen this chart posted:

kbps lowpass
8, 2000
16, 3700
24, 3900
32, 5500
40, 7000
48, 7500
56, 10000
64, 11000
80, 13500
96, 15300
112, 16000
128, 17500
160, 18000
192, 19500
224, 20000
256, 20500
320, 21000

Here by a xclame dev.

Not to mention that LAME would remove a bunch of really high frequency information even without an explicit lowpass filter, since it's not very audible(or completely inaudible) in most cases.

That chart pretty much matches what I hear. With AAC, it might be a bit better.

Quote:

Originally Posted by chewy4 

It seems to be hard to get concrete information on this. I've seen this chart posted:

 

kbps lowpass
8, 2000
16, 3700
24, 3900
32, 5500
40, 7000
48, 7500
56, 10000
64, 11000
80, 13500
96, 15300
112, 16000
128, 17500
160, 18000
192, 19500
224, 20000
256, 20500
320, 21000

 

Here by a xclame dev.

 

Not to mention that LAME would remove a bunch of really high frequency information even without an explicit lowpass filter, since it's not very audible(or completely inaudible) in most cases.

Just tried a few tests to confirm the above chart.  Assuming those are rates for stereo files, I find the LPF filter to be considerably higher than listed for low bit rate files using the mp3 encoder in iTunes, with spectrum analysis in Audacity.  For example, the 16K rate is shown as 3700, my test file made it up to 4500 before the roll-off starts.  Not a big deal, really, but when looking at bit rates that result in 20KHz response it looks like you could us lower rates than shown before loosing anything below 20KHz.  Probably depends on the specific codec, no idea which one is in iTunes, but I thought it was interesting. Also, it looks like at low rates mp3 has a higher top end than AAC, at high rates it doesn't seem to make any difference.

RIght after I posted the above I realized this could be entirely program dependent, because of how perceptual coding works. I might have just found a bit of music with highs that won't mask well, so they get coded instead of removed.  Looks like we'd need a group of files, and an average.  Sorry.