Audio Spectrum from Stereo to Mono

[KerimF]

Hi all,

After combining two stereo channels to mono, I used noticing a reduced level of the upper part of the original audio spectrum. For instance, this occurs when using other audio editors as well.

Let us suppose the stereo file is of *.wav type. It seems, so I may be wrong, the process of combining is to take the average of every two samples having the same timing and saving it as one sample to form the new mono channel. Actually, the original file length (excluding the wav header) will be reduced to half.
Speaking logic, less samples implies less information. So, perhaps, this could explain the loss, to some extent, of the higher frequencies when using this intuitive algorithm.

In my free time, I will try, by writing a small exe program, to combine the two stereo tracks by interleaving their samples to form one mono track which will have the original length. But, in this case, the bit rate, in the header, should be doubled to preserve the original audio spectrum (besides specifying there is one channel only).

What do you think? Thank you.

Kerim

[steve]

After combining two stereo channels to mono, I used noticing a reduced level of the upper part of the original audio spectrum.

I don't see that myself:

tracks000.png (575.82 KiB) Viewed 410 times

I do see that the spectrum is 6 dB lower than the stereo version (as expected), but it appears to be 6 dB lower at all frequencies:

Stereo

window-Frequency Analysis-000.png (42.06 KiB) Viewed 410 times

Mixed to mono

window-Frequency Analysis-001.png (49.65 KiB) Viewed 410 times

[KerimF]

Hi steve,

Thank you for your prompt reply.

From what you presented here there is no doubt that the audio spectrum is indeed preserved.

The difference I mentioned earlier is based on comparing the 'hearing' of the mono track and its original stereo channels.
I am 69 and I have one active ear only. This is why I have needed lately listening to hi-fi mono instead of stereo.

Now, I wonder if the difference I used noticing (with hi-fi music) will disappear if I will restore the global 6 dB loss.

[DVDdoug]

In my free time, I will try, by writing a small exe program, to combine the two stereo tracks by interleaving their samples to form one mono track

Somehow... I don't think that's valid because an amplitude difference between the channels becomes a "sudden" (high frequency) difference as you alternate between channels.

But, in this case, the bit rate, in the header,

The sample rate. WAV files are already interleaved so you can just change the sample rate & number of channels.

[KerimF]

Hi Steve and DVDdoung,

Would you please hear the two attached sounds?
“S_RoadToHell_ChrisRea1.wav” is selected from the original stereo channels
“M_RoadToHell_ChrisRea1.wav” is selected from the generated mono channel.

To my right ear in the least, the treble signal is not as clear in mono as it is in stereo.

Thank you,
Kerim

[steve]

In the stereo version, the sound has been processed in a way cause a very wide stereo spread, except for the bass frequencies which are essentially centre panned mono. Of course, in the mono version, the entire sound is centre panned mono. I think it's that difference that you are hearing.

[KerimF]

by steve » Fri Feb 01, 2019 1:14 pm
In the stereo version, the sound has been processed in a way cause a very wide stereo spread, except for the bass frequencies which are essentially centre panned mono. Of course, in the mono version, the entire sound is centre panned mono. I think it's that difference that you are hearing.

Do you mean you didn't hear the difference (using the right or left side of a headphone)?

On my side, I was expecting hearing all instrument's sounds combined in one track and without noticeable loss.
And I agree with DVDdoug that reading the stereo interleaved samples as of one mono track is not a good idea, even if the two stereo tracks are not very different.

This led me to think about another algorithm

Let us suppose L1, L2, L3... Ln the stereo samples of the left track (wav), and R1, R2, R3... Rn of the right one.
We may generate the samples Mx for mono as:

M1 = ( L1 + R1 ) / 2
M2 = ( R1 + L2 ) / 2
M3 = ( L2 + R2 ) / 2
M4 = ( R2 + R3 ) / 2
...
M(2x-1) = ( L(x) + R(x) ) / 2
M(2x) = ( R(x) + R(x+1) ) / 2
...
M(2n-1) = ( Ln + Rn ) / 2
M(2n) = Rn / 2

This will preserve the original audio file length. But, as I hope, it will preserve its contents better.

[steve]

Do you mean you didn't hear the difference (using the right or left side of a headphone)?

I mean that the "frequency content" (the frequency spectrum) of the mono track, is almost identical to the frequency content of the stereo track. But in the stereo track, there is a wide stereo spread of the higher frequencies, but as that is a stereo effect, it is only present in the stereo version.

I expect what they have done is to apply a stereo delay effect to the high frequency sounds.

To hear that the frequency content is actually unchanged, split the stereo track into two mono tracks (see: https://manual.audacityteam.org/man/spl ... racks.html), and then compare these two mono tracks with your "mixed to mono" track.

[steve]

I'm not clear what the purpose of this is. If it is that you want a mono mix that sounds brighter than a simple mono mix, then you could just mix to mono, then apply the Equalization effect to make it sound brighter.

Note that if you do that, you may need to Normalize and then apply the Limiter effect so as to bring the volume up to match the original (the original has been highly compressed to make it sound as loud as possible )

[KerimF]

Thank you, Steve, for the valuable hints.