Fixing Mono Tapes & Almost Stereo Recordings
Or Changing phase
by John M. Clement, PhD
Tapes and some records have the problem that the two channels may be shifted in time with respect to each other. If you wish to get optimum noise reduction you must combine the two channels to restore the original mono signal. But when you do this the highs may be muted because the two channels are out of phase. Fixing this problem is easy if your program has a time delay processor. Often this feature is included in other sections such as a reverberation simulator or stereo processor. If your program does not have such a feature, but it does have Direct-x plug-in capability, you can buy the Virtos Stereo Processor at a modest cost to perform this operation. The following instructions show how to correct time shifts using Magix Audio Cleaning Lab V3.0 and the Virtos Stereo Processor. If you have ACL 2004, then it can be done directly by ACL, and instructions for this are included below. The links in the text allow you to hear the files.
Here is an example below of an old Glen Miller recording which was purchased on a cassette tape. The wave was enlarged using the + button to expand the image. You can see that the peaks do not line up on both channels. The right channel is to the right of the left channel by about 4 bins.
To do this first click on the Edit button, then on DirectX
Plug-Ins as shown on the right
.
Next find the Virtos Stereo Processor and click on it as
shown below. Then click on Insert the selected Plug-in. 
The Virtos menu will appear. You must then rotate the stereo broadening to 0%. Then set the Time Offset. It can be set in 0.01 ms steps up to a maximum of 0.18 ms. Each bin is about 0.025 ms of delay. To fix the problem the right channel must be shifted backwards by -0.90 ms. Negative values shift the right channel left, and positive values shift it to the right. And important trick is to always round up the calculated value. If the calculated value is .023 then set the processor to .03. Since our calculated value was -0.09 you set the processor to -.10.
Now click on the X in the upper right corner of the Virtos Stereo Processor, and finally click OK on the DirectX plug-ins menu. The time shifted wave is shown below.
When you listen to the result in stereo you will notice that the image has been shifted slightly to the right. However if the two channels are combined into mono, the difference in sound is quite dramatic. The combined original wave is much duller sounding than the fixed and then combined wave.
When trying to align a tape recording you will often find that the amount of time delay varies from one end of the tape to the other. As a result you should adjust the delay individually for each track. In ACL this means you must split the wave file at track markers into separate sections and then individually adjust the delay for each section.
When you combine the two waves into mono, any noise that appears in one channel only will be reduced in the output. As a result the combined mono track will be quieter than the original. Also noise in mono is much less obvious than stereo noise. When the music appears to come from the front, but noise is heard on the side, the noise seems to be much worse. By combining the two channels both the noise and music come from the same location, so the music tends to mask the noise. This noise reduction by combining the two channels has no effect on the sound other than reducing the noise, so you will want to do this for all mono recordings, as long as the waves are properly aligned.
Combining the two channels is easy to do in Audio Cleaning Lab. You click on the Stereo FX button and adjust the sliders as shown in the picture below. Slide the stereo slider to mono, and slide the two level controls down until they are at -3.0 db. If you do not slide the level controls down, the signal will be greater and might be overloaded as a result.
Some stereo recordings may have very little separation between the right and left channels, but may have significant phase shifts between the channels. When these recordings are played over mono equipment they may sound very poor, as some frequencies are being cancelled. This problem will typically show up when the recording is played to a large audience, building PA system, or in recreational dance groups where the stereo has been combined to one mono channel. These recordings will still sound fine on conventional stereo systems. Such recordings must also have the two channels brought back into phase if they are to sound good under all conditions. At one time all commercial recordings were carefully monitored in both stereo and mono, and when necessary the phase of the two channels was adjusted for good performance under all conditions. However, home recordings and many amateur recordings have not been monitored, so you may have to also use this technique on these recordings.
Out of Phase Recordings
Occasionally it is possible to have a stereo recording when one channel has been recorded out of phase with the other. When this happens the recording will have a very “diffuse” stereo image and the sound will almost totally disappear when you play it in mono. You can recognize such a recording because the two channels are mirror images of each other as shown below.
This problem is also easily solved by the Virtos Stereo Processor. Adjust the processor for zero Time Offset, zero Stereo Broadening, and move the Right In slider all the way down until it reads -100%. This will flip the right channel wave to make it match the left channel.
Here is a handy table of Time Offset Values.
|
Bin Shift |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
|
Time offset |
0.03 |
0.05 |
0.07 |
0.10 |
0.12 |
0.14 |
0.16 |
0.18 |
If the right cannel peaks are to the right of the left channel, use negative values.
If the right channel is shifted more than 8 bins, you will have to use the processor several times until the two channels match. I have seen shifts as large as 18 bins in a few recordings. I suspect that such large delays were introduced by the record producer to simulate stereo, but they mess up mono reproduction and do not really sound like stereo.
Note: If you make recordings at 48 kHz rather than the standard 44, the Time Offset will be about 0.021 x the number of bins offset.
Since many audio restoration programs have the capability of delaying one channel, this idea can be used by other programs.
ACL 2004 has added a nice feature that allow you to edit each channel of a stereo recording separately. On the Edit button click on Split stereo channels. Now your stereo file has been split into 2 mono tracks, which act like stereo. If you need to delay one channel, just grab the wave and move it the necessary number of bins to the right or left. When done you can recombine the tracks into stereo by clicking on File, Apply all Realtime Effects.
Fixing Noise in FM Stereo broadcasts
Using the Virtos Stereo Processor along with another program it may be possible to remove noise in stereo broadcasts. Most of the noise is in the stereo signal and if you put the signal into mono, you will find that there is usually very little noise. The best way of processing the signal would be to only use noise reduction on the stereo or L-R signal. If you have a program that can apply noise reduction to just one channel then it is possible to do this.
First convert the original signal to L+R and L-R channels. To do this Set the two left out sliders to +50%. But set the two right out sliders to Left in +50% and Right in -50%. Then apply the noise reduction to only the right channel. Finally, process the signal with Virtos again with the same settings as the first time. The result should have dramatically reduced noise with very few audible problems. The main problem will be that the stereo separation may be somewhat reduced.
Even if your audio processing program can not apply noise reduction to just one channel, this method of processing may give better results than conventional noise reduction. This is now possible in ACL as the two channels can be split into separate tracks, and then recombined.
Open Reel Tapes
Open reel recordings generally came in 3 forms, full, half and quarter track formats. The full track was a strictly mono format, but half track could be in stereo, recorded in one direction or it could be in mono recorded in both directions. Quarter track was usually strictly stereo. Each type of tape should be played on a machine with the appropriate heads designed specifically to reproduce that format if you want good fidelity. Quarter track machines can play all three formats, but half track mono will reproduce with the correct sound in the left channel, and backwards sound in the right channel. In addition the head will not line up well with the track causing lost frequency response, uneven volume in the sound, and increased noise. Some quarter track machines had a shiftable head which could center the head properly for half track playback. This arrangement was satisfactory, but the lowest noise still was not possible.
If you only have access to a quarter track machine, you can easily unplug the unwanted channel, and use a tee to supply the same signal for left and right when playing back a half track mono tape. Some programs allow you to create two identical stereo tracks by setting the right channel to zero, and then mixing the left channel into the right channel. Some programs have difficulty doing this. For example ACL 3.0 has a Stereo FX processor which can be set to mono, but the right channel can only be reduced by 50db, and can not be eliminated. This is easily fixed by the Virtos Stereo Processor plug-in. You can set both left input sliders to 100%, and both right input sliders to 0%, with all other controls at zero. This removes the right channel and creates a mono file with only the original left channel in both channels.
ACL 2004 can do this process directly by using the Split Stereo Channels option. This will split the file into two separate files.
On some open reel machines it was possible to record each track independently and create quarter track mono. When played back such tapes will have separate programs on the right and left channels. Again, you can recable to fix this problem. Or you can record the entire tape, and then in a few minutes it is possible to split the channels using your software. The latter solution will save the time to replay the tape for each channel.