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crapfromthepast
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| Posted: 27 May 2022 at 2:57pm | IP Logged
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Out of curiosity, I picked apart some lossless stereo files of some mono songs from the early 1970s. I was curious if there was any benefit of saving them as 1-channel (true mono) lossless files, rather than 2-channel (stereo) lossless files where the channels have pretty much the same content.
I'll give you the answer first: For a mono song, I found a benefit to saving one track of the stereo file as a 1-channel ("mono") audio file.
Then, methodology: To look at each track, I perform an out-of-phase-sum test, or OOPS.
The process is straightforward in Audacity. Import a stereo audio file, use "Split Stereo to Mono" in the little rectangular menu at the left edge of the track, invert the right channel (by selecting the now-mono track corresponding to the right channel, then Effect > Invert), and hit play. If there's a volume imbalance between left and right channels, you can compensate for it by adjusting the volume one of the two now-mono tracks. If the song is mono, you'll get some nice cancellation between the left and right channels, which should leave behind some non-musical artifacts (like dithering noise).
(To make sure that Audacity doesn't introduce dithering artifacts of its own, go under Edit > Preferences > Quality, set the Default Sample Format to 16-bit, and set the High-quality Conversion Dither to None. Do this before you start.)
Dithering noise is noise that is intentionally added in software to randomize the smallest bit(s) of an audio signal. In general, it improves the sound at very low volumes (for CDs, that can be from around -90 dB to around -70 dB). Audio editing programs usually add it when they perform some kind of volume adjustment. For most recording/mixing/editing applications, you don't need to pay attention to it; programs like Audacity automatically add in dithering noise as a default setting. Wikipedia has extremely detailed entries for "Dither" and "Noise shaping" - it will surely tell you more than you need to know.
I found that the vast majority (maybe 80%) of good-sounding CDs with mono tracks, as well as just about all homemade two-channel audio tracks made by some superb audio people on this board, have left and right channels that don't sum to zero, but instead sum to dithering noise. Put another way, the left and right channels aren't digitally identical, but differ by an amount that represents dithering noise. The dithering noise can vary from track to track, from about -90 dB to about -40 dB.
Weird! I would have guessed that mono songs (with good mastering) would have left and right channels that would be digitally identical, but most don't. I did find some that were digitally identical (like the Grass Roots' The Complete Original Dunhill ABC Hit Singles CD from 2014), but this was a tiny minority, maybe 10% of everything I looked at.
To see what effect the dithering noise had on file size, I took a CD-ripped track and saved it as a stereo (2-channel) FLAC file. Then, in Audacity, I saved only the left channel as a true mono (1-channel) FLAC file. (Technically, I saved both as WAV, then used the same software and the same settings to data-compress to FLAC.) I compared file size (or, equivalently, data rate) for the two-channel and one-channel files. For tracks that sum to dithering noise, I found that the one-channel FLAC files are about 20% to 25% smaller than the two-channel FLAC files. Put another way, the FLAC encoding spends 20-25% of its effort to properly encode the dithering noise.
I also found that for a track where the left and right channels are digitally identical, saving just the left channel of audio as a 1-channel FLAC file has exactly the same file size and bitrate as the original 2-channel FLAC file. That's a good sanity check.
Interesting.
And it got me thinking: For most applications, like playing a CD, no one cares that there's dithering noise between the two channels. But for some applications that have a limited bandwidth, such as a streaming radio station that streams at 128 kbps, there could be some negative effects from the dithering noise. Basically, 20-25% of the bandwidth is taken up by encoding dithering noise, which nobody really cares about for mono songs. Imagine if we freed up that 20-25% to better encode just the audio we do care about. Doing so would likely improve sound quality. We can do just that by using just one channel of the song (I chose the left channel), and saving it as a 1-channel audio file.
Using just one channel from a mono song also eliminates artifacts that arise if the left and right channels are out of synch. I found one example - the mono song "Hearsay" by Soul Children, from the Time-Life CD Rhythm & Blues Vol. 20 1972 (1996), also released as Solid Gold Soul Vol. 7 1972 with the same mastering. "Hearsay" has the left and right channels out of synch by about two samples and a small volume difference between left and right channels. Here, saving just the left channel as a 1-channel FLAC file cut the file size roughly in half, meaning that the FLAC encoding devoted about half of its bandwidth to ensure that the left and right channels are reproduced properly (meaning, out of synch!) For synchronization errors, saving just the left channel as a 1-channel FLAC file really improves the sound in a tangible way, by eliminating the desynchronization problem.
For my own library (hand-picked tracks for every version of every song), I'm going through the folders and manually creating 1-channel FLAC files for every mono song, using the left channel from the 2-channel CD rip or homemade file. I estimate that I have about 1,000 files left to work through.
(If you're going to attempt this, don't just do a mass file conversion to "mono"; I think doing so will sum the left and right channels, which will hard-bake any desynchronization errors into the 1-channel file.)
I'm curious if anyone else has dug into this, or attempted to use 1-channel FLAC files for mono songs.
Edited by crapfromthepast on 27 May 2022 at 3:01pm
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AdvprosD
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| Posted: 27 May 2022 at 4:06pm | IP Logged
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This will be an interesting read as folks chime in to comment about their results.
For me, I am now for the first time ever, learning about dither in a digital recording. I would have guessed that it was an unwanted effect but read that's not the case.
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mjb50
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| Posted: 27 May 2022 at 7:11pm | IP Logged
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The vast majority of dither on ordinary CDs prior to around the 2000s was just a normal artifact of playing a mono tape through stereo gear. That is, the "dither" in those recordings was not generated noise that's added during digital processing, but rather, just the random noise that's inherent to the tape format, as well as in the electrical circuitry in the chain of audio gear in between the tape and the ADC. That's why there's rarely a CD where a mono recording manifests as bit-for-bit identical left & right channels. Also, as Ron noticed, sometimes there's a sync difference of 1 or 2 samples; that was probably introduced by the digitizing gear (I think the classic Sony PCM "adaptors" in the 1980s did this)...it's not enough to hear, but it's not perfectly mono.
As mastering tools became more sophisticated in the mid-1990s and beyond, more mastering was done in higher bit depths, which was then converted down to 16-bit for CD with the addition of digital dither. On mono recordings, sometimes this dither is in stereo, sometimes in mono. It's very inconsistent. Any random noise, be it tape hiss, electrical circuit noise, or dither, is very hard to compress losslessly. The more intense and stereophonic (different in each channel) it is, the more it will bloat the bitrate.
Dither is mainly for reducing bit depth; it eliminates quantization noise and allows for what is effectively a lossy version of the very quietest audio to exist "in the noise". However, it's still noise and is not ideal in all situations. I avoid using it if all I'm doing is cutting and pasting. As Ron indicates, Audacity's default settings add dither when saving, and it's better (IMHO) to turn this off and only use it when you know you want it.
As for the method of generating pure 1-channel mono from a recording that's mono-with-stereo-hiss, that's kind of a can of worms. When you sum the channels, you effectively reduce the hiss and increase the SNR by 3 dB. But if the signal is better in one channel, or there's a sync issue, as Ron says, it's best to just pick one channel. You have to decide track by track.
There's also an issue which has come up for me: some music was recorded with imperfect mono, where particular instruments or the background hiss associated with just those tracks on a multitrack (or channels in a mixer) might have a slight affinity for the left or right, and ultimately the result is a very narrow stereo mix. Accidental as it may be, it just doesn't sound the same when forced into pure mono. Should such music be treated as mono at all?
Now for the million-dollar questions: Do you really want to go to the trouble to permanently process an entire lossless collection in this way, forcing pure mono on the imperfect-mono tracks, just to save some space, even though disk space is cheap? And will you be keeping the originals just in case? And is this really the best way to go about it, or could you instead just tag the files to indicate what processing you want upon playback? (e.g. using foobar2000 with the Dynamic DSP component.)
For me, after experimenting 7 years ago with making some 1-channel mono versions of some rips, and running into these issues and philosophical questions, I decided to just live with the imperfect mono for now. I'm more interested in curating my collection and getting the best-sounding masterings. Worrying about the ideal processing of mono is much lower priority.
But to answer the general question, yeah, I've thought about this topic quite a bit. :)
Edited by mjb50 on 28 May 2022 at 11:42am
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crapfromthepast
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| Posted: 27 May 2022 at 7:21pm | IP Logged
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mjb50 wrote:
| Now for the million-dollar questions: Do you really want to go to the trouble to permanently process an entire lossless collection in this way, forcing pure mono on the imperfect-mono tracks, just to save some space, even though disk space is cheap? And will you be keeping the originals just in case? |
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Oh, I'd never trash the original CD rips to do this.
These 1-track files are for my radio and live DJ work. Space isn't the issue here; I see an advantage in no longer having to deal with desynchronization errors in some files.
The files end up looking like this - an example from my 1972 folder:
Undisputed Truth - Papa Was A Rollin' Stone [mono] {left channel of Complete Motown Singles Vol. 12A 1972 Disc 4}
The original rips are fully intact in a separate folder.
As for what's best, I'm making things up as I go. Aren't we all?
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AndrewChouffi
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| Posted: 28 May 2022 at 5:34am | IP Logged
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Thank you to all in this thread for explaining this to me.
This topic has always been interesting to me (even though
I've always been somewhat uninitiated to do any
research...)
Andy
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mjb50
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| Posted: 28 May 2022 at 12:12pm | IP Logged
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Quote:
| Undisputed Truth - Papa Was A Rollin' Stone [mono] {left channel of Complete Motown Singles Vol. 12A 1972 Disc 4} |
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Nice! I really appreciate that you keep track of the provenance and what customizations you made. I've run across so many fan reconstructions & edits, not just in the wild but things I did myself in years past, where it's a total mystery how I ended up with this slightly different file, and I wish I knew now.
I've gotten better over the years at making notes, though. For example, I have a lot of files now with comments in them like this:
GED 24081 / GEFD 24081 [1991 CD; track boundaries corrected after rip]
When it comes to mastering problems, I'm often just leaving notes in the filenames and thinking "I'll deal with this later", e.g.:
Billy Ocean - Caribbean Queen (No More Love on the Run) (Extended Version) [8m09s; LR balance off]
I use that foobar2000 component for some mastering problems, so that when the file name (or a tag if I prefer) contains a certain phrase like weak bass, weak treble, excessive treble, channels swapped, slightly fast, or slightly slow, an appropriate DSP chain is applied to fix the problem(s). Results are not always perfect, but improved. I also use foobar to do format conversions for my portable devices, and it lets me apply that same processing, so the converted files have the fixes.
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NightAire
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| Posted: 28 May 2022 at 10:25pm | IP Logged
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Absolutely fascinating! I never tried this and would never have guessed the dither would increase the file size that much.
I've converted 2-channel "mono" files to both = left or both = right but in saving with dither again I may have just restored the excess data I was trying to eliminate.
Dither isn't specifically a stereo tool as I understand it, so it's interesting that a stereo 2-track would create a different dither pattern in supposedly identical channels. A mono channel should have dither, too.
The only thing I can think is that dither is supposed to be random, so maybe that's why they're different?
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Hykker
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| Posted: 29 May 2022 at 5:30am | IP Logged
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Can't say as I really understand some of this discussion, especially when it concerns dither and exactly what it is/does.
I will say that when I'm digitizing a mono vinyl source I'll do it as a mono file (using Adobe Audition 1.5), with the source mono'd
out (audio goes thru a Mackie mixer). Sometimes if the source is less than pristine I'll take one channel or the other (or something
other than a 50/50 mix). Is this not a good idea?
I realize that it would make it more difficult to do noise reduction, eliminating anything not common to both channels.
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crapfromthepast
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| Posted: 29 May 2022 at 8:36am | IP Logged
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Gene - I was surprised to see that the dither is in stereo. I could try to rationalize a reason for it, but I don't fully understand why it's in stereo either.
Hykker - The steps you're taking are perfect. I think if you just save your final mix as a mono file (1-channel), rather than a stereo file (2-channel), you'll get the benefits of all the steps you're taking. (I think Audition has a "mix to mono" option, or something like that. Cool Edit Pro had such an option.)
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aaronk
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| Posted: 29 May 2022 at 11:15am | IP Logged
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In a nutshell, dither is very light noise that is added during digital audio editing and processing. In general, the noise is so low that you'd never hear it while the music is playing. Sometimes dither noise is only generated in frequencies outside of our hearing range (i.e. above 20kHz). For dither noise that covers the entire frequency spectrum, the only time you can really hear it is if you crank up the volume when playing the tail end of a digitally faded file. If you've ever created a fade out on an audio file without dither, you might notice that the very end of the fade sounds weird and digitally distorted (where the audio is almost becoming inaudible). Dither adds very light noise and keeps this super low audio from becoming distorted.
This is a simplified explanation, and the higher the bit depth of an audio file, the less important it is to apply dither. It's usually used on 16 bit files (i.e. CD quality, ~96dB of dynamic range). As the bits go up (24 bit, 32 bit), the more dynamic range that file can have (~144dB dynamic range and higher), and therefore super low volume audio won't be distorted like it is with a 16 bit file.
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The Hits Man
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| Posted: 01 June 2022 at 2:07pm | IP Logged
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I never mono-ize anything. I keep everything stereo for
standard and convenience purposes.
As for dither, I use flat dither. I find it sounds best
for preserving high frequency transients and bass. I
never use noise shaping, which so many CDs, especially
from the 90s, use when it became fashionable.
If I create a file that i've dithered to 16-bit with flat
dither, only to have to go back one day and edit it in
some way, I can avoid strange noise anomalies from having
edited a noise-shaped file.
I've found that noise-shaped files alter the timbre and
clarity of the music. That's another reason I don't use
it.
I use MBIT+ dither within iZotope RX and Ozone at the
normal settings, no noise shaping. I find that most of
the time it works very well for my purposes.
I often use a spectrum analyzer to see exactly where that
noise is going and at what frequencies it falls into at
silence, and where it peaks. For 16-bit files, as long
as I keep it below 98db, i'm good. I also do this in
case I do have to re-process a file. Someone will
correct me if I am wrong, but, with iZotope RX, it allows
me to re-dither only parts of a file that need it with
the "Only When Quantized" setting.
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The Hits Man
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| Posted: 01 June 2022 at 2:12pm | IP Logged
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Oh, for files that aren't completely mono, meaning they
may have a slight channel imbalance, often means that a
two-track deck was used to play back the mo0no tape, and
the engineer didn't adjust this. When I encounter this
coming off a CD, I correct it. There can sometimes be
slight phase issues. To simply reduce such a file to
mono could create a baked-in phase distortion that can be
audible.
One way to eliminate any channel/phase imbalances is to
pict the best channel, and copy it to the other channel
for a perfect mono file that plays stereo, and can be
cleanly folded to mono without issues.
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VWestlife
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| Posted: 02 June 2022 at 5:03pm | IP Logged
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The main problem is that when CDs were made from mono source tapes, they were often played using a stereo tape head, and unless the head was aligned perfectly, you're going to end up with slight differences between the left and right channels, such as one channel being louder and/or brighter than the other, or the two channels being slightly out of phase with each other.
If you then sum the two channels together to mono, you may end up with undesirable artifacts, such as a loss of treble response, or an odd "phasey" effect, that may be constant throughout the recording or may come and go, producing an unpleasant and distracting effect.
So unless you have the time to go through each track one-by-one and try to precisely align the two channels before summing them to mono, or pick the one channel that sounds better and ignore the other one entirely, it's best to just leave the file as "stereo", even if there's virtually no difference between the channels.
As already mentioned, if the two channels are exactly identical, a lossless codec like FLAC will recognize that, and the resulting file size won't be any larger than a single-channel recording.
And with lossy codecs like MP3 and AAC, encoding it as 320 kbps stereo instead of 160 kbps mono, for example, will definitely yield higher quality, because the bitrate is not exactly halved between the two channels; even with mid/side encoding disabled, they're still allowed to share the bitrate reservoir between the channels, allowing greater encoding efficiency. In iTunes, I've noticed that encoding a pure mono recording as 160 kbps mono had some noticeable artifacts that did not exist when encoding it as 320 kbps stereo.
Plus, then you have to take into consideration mono recordings that were doctored to create a simulated stereo effect, by adding reverb, putting more bass in one channel and more treble in the other, slightly delaying one channel, or using comb filtering to "spread out" the sound. These effects can all produce undesirable artifacts when you sum them to mono.
And as already mentioned, there are some recordings which may sound like mono at first glance, but actually do contain some degree of genuine stereo effect, such as CCR's "Proud Mary", the Raspberries' "Go All the Way", Modern English's "I Melt With You", etc.
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The Hits Man
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| Posted: 03 June 2022 at 11:54pm | IP Logged
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VWestlife wrote:
The main problem is that when CDs were
made from mono source tapes, they were often played using
a stereo tape head, and unless the head was aligned
perfectly, you're going to end up with slight differences
between the left and right channels, such as one channel
being louder and/or brighter than the other, or the two
channels being slightly out of phase with each other.
If you then sum the two channels together to mono, you
may end up with undesirable artifacts, such as a loss of
treble response, or an odd "phasey" effect, that may be
constant throughout the recording or may come and go,
producing an unpleasant and distracting effect.
So unless you have the time to go through each track one-
by-one and try to precisely align the two channels before
summing them to mono, or pick the one channel that sounds
better and ignore the other one entirely, it's best to
just leave the file as "stereo", even if there's
virtually no difference between the channels.
As already mentioned, if the two channels are exactly
identical, a lossless codec like FLAC will recognize
that, and the resulting file size won't be any larger
than a single-channel recording.
And with lossy codecs like MP3 and AAC, encoding it as
320 kbps stereo instead of 160 kbps mono, for example,
will definitely yield higher quality, because the bitrate
is not exactly halved between the two channels; even with
mid/side encoding disabled, they're still allowed to
share the bitrate reservoir between the channels,
allowing greater encoding efficiency. In iTunes, I've
noticed that encoding a pure mono recording as 160 kbps
mono had some noticeable artifacts that did not exist
when encoding it as 320 kbps stereo.
Plus, then you have to take into consideration mono
recordings that were doctored to create a simulated
stereo effect, by adding reverb, putting more bass in one
channel and more treble in the other, slightly delaying
one channel, or using comb filtering to "spread out" the
sound. These effects can all produce undesirable
artifacts when you sum them to mono.
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Yeah, I basically said the very same thing above.
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Topic: mono songs saved as 1-channel audio files
