M0LTE.FmChannel 0.4.0

M0LTE.FmChannel

A physical FM link simulator, for measuring what a soundcard modem really does over an FM voice path.

Nothing about the FM impairments is approximated by a curve. The modem's audio is genuinely frequency-modulated onto a carrier, complex noise is added there at a stated carrier-to-noise ratio, and a limiter and a discriminator bring it back through the radio's real audio paths. The threshold effect, the discriminator's triangular noise, pre/de-emphasis and the band limits all emerge from that, rather than being asserted.

using M0LTE.Fm;

var link = FmLinkProfile.MicAndSpeaker(peakDeviationHz: 3000);
var channel = new FmChannel(link, audioRate: 48000, seed: 1);

float[] heard = channel.Apply(transmitted, cnrDb: 20);

The namespace is M0LTE.Fm, not M0LTE.FmChannel: a type called FmChannel inside a namespace of the same name makes new FmChannel(...) ambiguous, and the compiler resolves it to the namespace. Package id and namespace are allowed to differ, and here they have to.

Why it exists

Because an FM link is not a linear channel, and measuring an FM mode against flat AWGN gives numbers that mean nothing. Three things fall out of doing it properly:

• The threshold effect. Well above threshold the discriminator suppresses noise and the output beats the input carrier-to-noise ratio. Below it, click noise takes over and the output collapses far faster than the input degrades. FM modes do not fade away gracefully - they fall off a cliff, and where that cliff sits is the number that matters.
• Triangular output noise. Discriminator noise power rises with the square of audio frequency, so a mode's high-frequency content is measurably noisier than its low. This is why pre/de-emphasis exists, and why a wideband audio mode cannot be masked honestly against flat AWGN.
• Emphasis and band limits are the channel. A microphone input's pre-emphasis and 300-3000 Hz passband are not incidental - for a mode designed to work through mic and speaker they define what is transmittable at all.

The carrier-to-noise convention

cnrDb is carrier power over noise power in the receiver's IF bandwidth (FmLinkProfile.IfBandwidthHz, about 8 kHz on a 12.5 kHz channel and 16 kHz on 25 kHz). That is where an FM receiver's threshold is defined.

It is deliberately not the SNR-in-3-kHz convention the HF/SSB world uses. The two are different quantities and a number moved between them without conversion would be wrong, so anything reporting results from this should say which it is quoting.

Link profiles

Everything is settable on the record directly - deviation, IF bandwidth, the audio passband at each end, the emphasis time constants, a deviation calibration error, and flat-Rayleigh flutter.

FmLinkProfile.IfBandwidthForSpacing() gives the IF bandwidth for a channel spacing, and the figures are a real radio's rather than a rule of thumb: 12.6 kHz wide, 12.0 kHz medium, 7.8 kHz narrow, from the Tait TM8100/TM8200 service manual MMA-00005-05 p.73 Table 3.1. Which spacing a mode belongs on is a property of the mode, not a preference.

Up to 0.3.0 this returned 16 kHz for anything at or above 20 kHz spacing and 8 kHz below, which is 27 % too wide on a 25 kHz channel and 33 % too wide on a 20 kHz one. Since the carrier-to-noise ratio is stated in this bandwidth, that moved every wide-channel measurement by about 1.1 dB, in the direction that flatters the modem. Wide and medium channel numbers measured before 0.4.0 are not comparable with numbers measured after it.

Filters are specified in hertz

A windowed-sinc's transition width is roughly rate/taps, so a fixed tap count makes a filter twice as sloppy each time the sample rate doubles - which is not how a radio behaves, its audio filters being analogue and no wider for a sound card sampling faster. Every filter here is therefore specified by its shape and the tap count derived: the transition is a fixed fraction of the passband, at whatever rate the stage runs at.

That matters because it is what makes a measurement at one rate comparable with one at another. It was not always so: up to 0.2.0 the tap counts were fixed, and the same waveform measured at two rates went through two different channels with the higher rate penalised. Nothing measured through 0.2.0 or earlier is comparable with anything measured through 0.3.0 or later.

Licence

GPL-3.0-or-later. Extracted from packet-net/pdn-soundmodem, which is GPL-3.0-or-later, so this is too and must stay that way. See COPYING.