correct compressed audio files

[Alexander]

 

The Denon avr-x3200w has a feature called the 'restorer', used to correct compressed audio files. There are three settings – low, medium & nigh of which I had always used medium for listening to Pandora. My question is has anyone used this feature and what setting have you used? Pandora uses 192k bps going through my system.

Edited July 24, 2020 by Alexander
Corrected Pandora is 192kbps not 128kbps

[OO1]

First time , I hear of such a feature ,  very interesting

[Chris A]

Quote

Restorer

Compressed audio formats such as MP3, WMA (Windows Media Audio) and MPEG-4 AAC reduce the amount of data by eliminating signal components that are hard for the human ear to hear. The “Restorer” function generates the signals eliminated upon compression, restoring the sound to conditions near those of the original sound before compression. It also restores the original bass characteristics for a rich and expanded tonal range.

Mode

High:	Optimized mode for compressed sources with very weak highs (64 kbps and under).
Medium:	Apply suitable bass and treble boost for all compressed sources (96 kbps and under).
Low:	Optimized mode for compressed sources with normal highs (96 kbps and over).
Off:	Do not use “Restorer”.

 

This item can be set with analog signals or PCM signal (Sample Rate = 44.1/48 kHz) is input.

This item default setting for “Online Music”, “iPod/USB” and “Bluetooth” is “Low”. All others are set to “Off”.

This cannot be set when the sound mode is set to “Direct” or “Pure Direct”.

“Restorer” settings are stored for each input source.

 

There are some clues here to what "Restorer" is doing.  MP3 and other lossy compression codecs completely truncate frequencies above about 15 kHz (based on the selected bit rate of the lossy compression algorithm).  Here is an example of a soundtrack from an old movie (Mysterious Island, 1961 by Bernard Herrmann).  The vertical scale is frequency, the horizontal is time, colors indicate loudness levels at each frequency:

 

 

As can be seen above, everything above 15 kHz is completely truncated.  If a lower bitrate lossy compression is used (smaller music files to stream to your devices), that upper frequency where the truncation occurs would be lower--perhaps 12 or 10 kHz.  The easy way to "Restore" this is to create a second harmonic of the frequencies above 7-10 kHz, thus artificially "restoring" those truncated frequencies.  This is what the "low", "medium" and "high"  Restorer settings are apparently doing. 

 

It looks as if Restorer is applying more artificial high frequencies that are generated when using the "high" setting, corresponding to lower bitrate music tracks--which have a correspondingly lower cutoff frequency of the music (as low as 7 kHz). 

 

Another effect that occurs with lossy compression is that the frequencies on either side of the fundamental frequencies of the musical instruments/voices are also truncated--leaving a "pure signal" without the "in between" frequencies.  One way to put back some frequencies (i.e., not the exact frequencies that were stripped out, but manufactured frequencies that probably are not representative of the frequencies that were removed) is to create modulation distortion with the signals that are there, such that the lower frequencies modulate the higher frequencies, thus adding "fullness" (otherwise called "opaqueness") to the resulting sound.  This is exactly what Klipsch loudspeakers are usually built to avoid--modulation distortion. This is apparently being done with the "low", medium and high Restorer settings.

 

A third effect is not really related to the lossy compression algorithms: EQing.  This type of added distortion is to perhaps apply some inverse EQ to the incoming signal to partially counteract the applied EQ to the tracks that both the lossy compression algorithm might have applied, and the mastering people applied to the tracks to unbalance the natural harmonics to make the sound more "pleasing" and perhaps a little less muffled due to the high frequency truncation. This is what your "medium" setting is apparently doing.

 

If you are worried about what setting to use, I recommend lossless music tracks, where all this guessing is eliminated, and you get to hear what the musicians put on the tracks to begin with.

 

Chris

[Edgar]

1 hour ago, Chris A said:

If you are worried about what setting to use, I recommend lossless music tracks, where all this guessing is eliminated, and you get to hear what the musicians put on the tracks to begin with.

 

A friend of mine said just the other day that he has to go all the way to 320 kbs or 384 kbs MP3 before he gets acceptable quality from 16/44.1 sources. I responded that FLAC and similar lossless algorithms will get him to approximately 750 kbs losslessly, so that extra 2:1 compression from MPEG is hardly worth the cost.

[Alexander]

 

Thank you for elaborating on this Chris A, all of that makes sense.

 

I do have two totally independent systems that run through the klf-30s. The Emotiva rack is for all of the lossless music and the Denon is used predominantly for HT and things like Pandora for casual or back ground music. Just was looking to improve on the Denon system if practical. The blu ray player in the Denon system I feel is just too low of a grade to use as a CD p;layer. Maybe someday I can get back into vinyl but that will just have to wait.

[Dave A]

@Chris A is there an equivalent "Restorer" feature in Audacity?

[JohnA]

1 hour ago, Edgar said:

 

A friend of mine said just the other day that he has to go all the way to 320 kbs or 384 kbs MP3 before he gets acceptable quality from 16/44.1 sources. I responded that FLAC and similar lossless algorithms will get him to approximately 750 kbs losslessly, so that extra 2:1 compression from MPEG is hardly worth the cost.

 

I agree.  I can often hear distortion in 256k tracks riding my motorcycle.  Even 320k destroys the stereo image, re: "Blackwater" by The Doobies.

[Chris A]

5 hours ago, Dave A said:

@Chris A is there an equivalent "Restorer" feature in Audacity?

I haven't seen one but I'm sure that there's a VST plugin (perhaps even freeware) that does approximately the same thing as Denon's Restorer.  Both Audacity and foobar2000 freeware player take VST plugins (i.e., if you add foo_vst to your foobar components folder). Since I don't use lossy format music files, I haven't had the need to look for ways to fix them.

 

I've used a couple of VST plugins--compressor/expanders--with both Audacity and foobar2000 (Transient Monster and Red Phatt Pro) to expand the dynamics of over-compressed files. 

 

Chris

[Marvel]

On 7/24/2020 at 11:02 AM, JohnA said:

Even 320k destroys the stereo image, re: "Blackwater" by The Doobies.

 

How would you know? 😜

 

I could send you a 320kb/s files that you would find hard to tell is an mp3.

 

On 7/24/2020 at 3:02 PM, Chris A said:

I've used a couple of VST plugins--compressor/expanders--with both Audacity and foobar2000 (Transient Monster and Red Phatt Pro) to expand the dynamics of over-compressed files. 

 

A bit tricky to do that right.

 

Bruce

[Chris A]

1 hour ago, Marvel said:

A bit tricky to do that right.

Really what is needed is a multiband compressor/expander, not a single-band compressor/expander like those I mentioned above.  The last time I looked, multiband compressor/expanders weren't freeware. 

 

One of the things you learn about undoing mastering compression and EQ is that it's much better to do it once using something like Audacity or Pro Tools...and be done with it.  I've also found that each track needs to be separately fixed (...if it is a popular/rock music track.  Classical is a different story.)  This means that you really need to look closely at each track.  There is no "automatic fixit" plugin that works at playback time: you actually need to put a little effort into demastering the track if it is going to sound right.

 

Chris

[Dave A]

Pretty serious discount on Pro Tools until the end of this month.

[Marvel]

1 hour ago, Dave A said:

Pretty serious discount on Pro Tools until the end of this month.

 

I'll stick with what I've got. It's like drinking Apple's kool aid...