New filter for de-Multiplexing recorded mono audio

[Noitarud]

Hello all I would like to tell of a discovery I made over ten years ago, it might have some novelty or serious use as a filter or even a codec.

I built a mono FM transmitter and once when I tested it I had nothing to transmit so I used an FM radio station, I received it on another radio – and the STEREO light came on!

Would you like to create a filter, since this would also work with mono VHS taped recordings of television:
•Decode mono-multiplex to stereo
•Subtitles may also be acquirable (FM? I do not have info for stereo, only quad (see Comment 6), FM radio should have some embedded things. For TV, they are probably using teletext, which could become another FFMPEG filter idea, again a novelty, but part of the stream).

Cons:
•I do not think Quad is in active use (despite modern cars being equipped with four speakers), so decode to quad is not necessary.
•Niche, and people may assume if acquired in mono there is no way you could get stereo.
•TV stereo may be implemented different to FM stereo, I have not yet tested Stereo TV modulators.
•I have not tested with VHS recordings or audio cassette to see if actually usable.
•Analog TV is outdated

(The rest is not related to ffmpeg, but would be useful elsewhere)

For these two functions (for retransmission to mono transmitter)
•Encode stereo to mono-multiplex
•Encode quadrophonic to mono-multiplex
Better to create an output filter (like WaveOut) or as FFDSHOW filter, so all MP3s and such can be played without file conversion.

And consider a codec, the monaural stream could confer benefits, and for the stream's decoder, have either full decode [or decode to mono-multiplex (for stereo or quadrophonic FM receivers) - not required if can Encode output sound, may assist troubleshooting].

[Elon Musk]

Are there some specifications for this algorithm(s) ?

[Noitarud]

Big changes

[Noitarud]

There is this:
​https://en.m.wikipedia.org/wiki/FM_broadcasting#Stereo_FM
It is important that stereo broadcasts be compatible with mono receivers. For this reason, the left (L) and right (R) channels are algebraically encoded into sum (L+R) and difference (L−R) signals. A mono receiver will use just the L+R signal so the listener will hear both channels through the single loudspeaker.
The (L+R) signal is limited to 30 Hz to 15 kHz to protect a 19 kHz pilot signal. The (L−R) signal, which is also limited to 15 kHz, is amplitude modulated onto a 38 kHz double-sideband suppressed-carrier (DSB-SC) signal, thus occupying 23 kHz to 53 kHz. A 19 kHz ± 2 Hz[12] pilot tone, at exactly half the 38 kHz sub-carrier frequency and with a precise phase relationship to it,…is also generated. The pilot is transmitted at 8–10% of overall modulation level and used by the receiver to identify a stereo transmission and to regenerate the 38 kHz sub-carrier with the correct phase. The composite stereo multiplex signal contains the Main Channel (L+R), the pilot tone, and the (L−R) difference signal. …Another way to look at the resulting signal is that it alternates between left and right at 38 kHz, with the phase determined by the 19 kHz pilot signal.

​https://en.m.wikipedia.org/wiki/FM_broadcasting#Quadraphonic_FM
There are two additional subcarriers in the Quadraplex system, supplementing the single one used in standard stereo FM. The baseband layout is as follows:

50 Hz to 15 kHz main channel (sum of all 4 channels) (LF+LR+RF+RR) signal, for mono FM listening compatibility.
23 to 53 kHz (sine quadrature subcarrier) (LF+LR) − (RF+RR) left minus right difference signal. This signal's modulation in algebraic sum and difference with the main channel is used for 2 channel stereo listener compatibility.
23 to 53 kHz (cosine quadrature 38 kHz subcarrier) (LF+RR) − (LR+RF) Diagonal difference. This signal's modulation in algebraic sum and difference with the main channel and all the other subcarriers is used for the Quadraphonic listener.
61 to 91 kHz (sine quadrature 76 kHz subcarrier) (LF+RF) − (LR+RR) Front-back difference. This signal's modulation in algebraic sum and difference with the main channel and all the other subcarriers is also used for the Quadraphonic listener.
105 kHz SCA subcarrier, phase-locked to 19 kHz pilot, for reading services for the blind, background music, etc.
The normal stereo signal can be considered as switching between left and right channels at 38 kHz, appropriately band-limited. The quadraphonic signal can be considered as cycling through LF, LR, RF, RR, at 76 kHz.[20]

[Noitarud]

Replying to Elon Musk:

Are there some specifications for this algorithm(s) ?

(See separate post)
So, can also extract subtitles from it too, though only stated for quad there probably is an equivalent for stereo…

[Elon Musk]

FFDSHOW filter is not related to this project.

[Noitarud]

Excuse me,
This idea is:Would you like to create a filter, since this would also work with mono VHS taped recordings of television:
•Decode mono-multiplex to stereo.

Television may be handled a little different to FM.
(And, to be specific, "new codecs" are not related either)

[Noitarud]

(For some reason it has disabled "reply")
Since opening, I have found it has been hard to clarify what I really want with this and I forgot your requirements, encapsulated irrelevant things to ffmpeg.

[Michael Koch]

There don't exist any records with a L-R signal on a 38kHz carrier. This is only used for FM transmission. It makes no sense to write a filter for it.

[Noitarud]

If my mono transmitter passed the carrier then any recorders should be able to capture the carrier and you will be able to decode to stereo, provided the recording equipment has the bandwidth to capture it. But I would assume the excess would be rejected by codecs so recordings shouldn't retain it.

[Noitarud]

…and provided they are retained on the cassettes. If I test any of my old VHS tapes I might post how it goes - Unless someone else wants to confirm this by doing a dub to the computer with the highest sample rate possible, if the carrier is in the Fourier then there are grounds for such a filter.

I will reopen for one more exposure of this, and if I don't test it will be the last reopen.

[Elon Musk]

No digital specifications provided.