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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 anywhere near 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).

Unlike DRM though, it never drops out!

Unlike ISB Stereo on shortwave, selective fading doesn't change the stereo balance.

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)

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 decoder has an unfortunate effect of tearing the mono signal apart into two channels. There is nothing left in the centre! This sounds abnormal, so the left and right channels are blended together at low frequencies to keep the bass and vocals more in the centre.

Where it does work very well is on noise-like sounds, such as snare drums, cymbals and other percussive sounds. Since they are wideband and not frequency critical, they are passed by the comb filters very well and produce a good stereo effect.

A recent test on TIAMS060 (at time 8m 11s) using a track with hard-panned vocals and instruments also produced very good stereo separation.

So it's 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 very 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

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  (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 v7.

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 April 16, 2020: RNEI on 6070kHz from Channel292 Germany This Is A Music Show on 5850kHz and 7780kHz WRMI Florida The KBC Aussie Song Request (Occasionally)

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 Rose from RNEI on March 30, 2020:

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