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Q: What is it?

A: Comb Stereo is a way of sending a stereo signal through a mono channel. Sort of. (It has limitations...)

Q: Is it real stereo?

A: Yes. It does send two channels separately, so it's real stereo. Sounds can be sent in the left or right channels in any combination.

Q: Is it good stereo?

A: Well.. No! Don't expect it to be as good as DAB, DRM, FM Stereo, AM Stereo (C-QUAM), or even ISB stereo (perhaps the best suited for shortwave, but not currently used).

But the other systems have problems...

DAB uses digital modulation and is transmitted on VHF. This causes reduced range compared to FM, and many services have a lower than ideal bitrate causing digital artifacts which reduce quality.

DRM (Digital Radio Mondiale) also uses digital modulation and is mainly used on Mediumwave (AM Band) or HF (shortwave). DRM can drop out (go silent) due to severe fading, severe noise, or interference. The bitrate is low, and this causes digital artifacts which reduce quality. There is a tradeoff between error correction and bitrate. Making the signal more error-proof reduces the bitrate and reduces quality.

FM Stereo causes a 23dB degradation in SNR compared to mono. This causes a big increase in hiss levels. FM Stereo also uses a much wider modulation bandwidth of 53kHz, which makes it more susceptible to multipath degradation than FM Mono.

C-QUAM AM Stereo suffers from "Platform Motion" during fading conditions, which makes the sound wander left and right. The SNR is slightly lower than mono AM.

ISB Stereo suffers from something similar to C-QUAM, in that selective fading changes the stereo balance - which also makes the stereo wander left and right. The SNR is slightly lower than DSB AM, because the sidebands are not coherent (in-phase).

Comb Stereo does not have these problems!

The bandwidth is the same as mono - So the SNR should be about the same as mono.

Selective fading doesn't affect the comb bands much, so the balance is largely unaffected by selective fading notches.

The Comb Stereo artifacts are much like typical music effects of echo, chorus, fast reverb or room reflections.

But Comb Stereo has some of it's own problems...

The Comb Stereo encoder is quite good, in that it's very compatible with mono listening. For the most part, there is no obvious difference in sound quality.

BUT.. The basic Comb Stereo decoder has a problem of splitting the sound into Left and Right ONLY. There is nothing in common with the two channels, so there is no "centre" to the stereo image!

The enhanced Comb Stereo decoder corrects this by special Left/Right processing to reduce separation on mono sounds and boost separation slightly on Left or Right only sounds. This produces a much more normal sounding stereo decode.

Comb Stereo encoding causes the peak levels to increase, which lowers the audio loudness. But since it is done before modulation processing this doesn't matter too much.

Comb Stereo has some strange characteristics...

On peak reading VU meters tested using white noise, the L/R separation appears to be only about 6dB. But the actual separation is much higher.

Comb Stereo divides the Left and Right channels into a "comb" of frequencies to send the stereo information, so each channel only has half the frequencies present. But mono/centre sounds are not encoded, so all the mono/centre frequencies are sent. Also, stereo difference (L-R) is not combed either! So "surround" sounds are also flat (above 200Hz). And, strangely.. it sends the stereo difference (L-R) as if it were mono L+R! (but delayed and phase shifted) This means surround sounds decode as centre sounds, so the system can't cope with more L-R than L+R signal.

The majority of sounds in music are in the centre, so this scheme is very compatible with mono. Most sounds are not encoded, and so the encoder output sounds mostly normal. Sounds that are only in the Left or Right channels are comb filtered, and may have a slightly "hollow/pipe" sound to them. However, our ears get used to hearing the comb filter artifacts because they happen every day - anywhere you have sounds reflecting off flat surfaces like walls, ceilings and floors.

The new enhanced v7e and v7f decoders feature a special multiband stereo expander to improve performance. The result is far more natural sounding stereo with much fewer artifacts, AND good separation.

Hear a test of the new decoder here on YouTube.

Comb Stereo is amazingly simple and effective - considering it's sending two channels over a single path, with no increase in bandwidth!

* Comb Stereo was rapidly developed from an idea in about 2 days, and is still in it's early stages. Expect more improvements!

Q: Great! So how can I decode it?

A: Download the decoder plugin from here, and feed your receiver audio into it using your PC:
Version 7 Comb Stereo Decoder - Includes Equalizer OLD VERSION

Version 7e Enhanced Comb Stereo Decoder with EQ IMPROVED v7e

Version 7f6 TIAMS Enhanced Comb Stereo Decoder with Auto-EQ NEWEST TIAMS/RNEI wide VERSION 7f6 - Updated September 4th, 2020. For TIAMS and RNEI on WRMI 5850kHz only.
Better stereo, more bass separation, less artifacts, selective fade reduction, bandwidth expansion for TIAMS, and flatter Auto-EQ.

Version 7f6 KBC/RNEI Enhanced Comb Stereo Decoder with Auto-EQ NEWEST RNEI/KBC narrow VERSION 7f6 - Updated September 4th, 2020. For RNEI on 6070kHz and KBC only.
Better stereo, more bass separation, less artifacts, selective fade reduction, and flatter Auto-EQ.

Or, use the combined AM Sync Demodulator and Comb Stereo Decoder! (for 0Hz IF IQ input, like KiwiSDRs):
AM Sync Demod plus Comb Stereo Decoder, DNR and Equalizer OLD VERSION (Enable "Comb Stereo Decoder" in settings)

* If the encoder is upgraded, you will most likely need a new version of the decoder. Check back here for updates!
Currently Encoder v7 is used. Any v7 Decoder will work, but v7f6 is the latest and best.

Please note:

Q: How can I run those plugins in real time?

A: Try VSThost:

Here's a guide from RNEI to setting up VSThost for Comb Stereo decoding:

Q: Who's using it? What programs can I hear in stereo?
Programs using Comb Stereo as of September 7, 2020: RNEI on 6070kHz from Channel292 Germany AND NOW ALSO 5850kHz WRMI Florida! This Is A Music Show on 5850kHz and 7780kHz WRMI Florida The KBC Aussie Song Request at about 0141UTC Sunday

Q: But I want to hear something NOW!

A: Well, have a listen to some of the newest recordings from shortwave here:

Some of them are in decoded Comb Stereo!

Q: So where did this idea come from?

A: From a chat with Roseanna from RNEI on March 30, 2020:

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