LittleSam129 a37ad1f7b3 Lab042: transmit a JPEG through a cable loopback ...

First lab in the project that switches the transmitter on. Lab001-Lab041
built the whole chain but never called into an SDR to send anything.

Two modes. Software closes the channel in memory and needs no hardware;
it is the reference the hardware run is measured against, so the
difference between them is what the radio path contributes. Hardware
drives one PlutoSDR with its transmit output looped back through a 30 dB
attenuator into its own receive input.

Everything outside the device wrapper is reused from protocol/ unchanged:
image_fragments, packet and the primitives in bpsk_radio.

Software run: a 4756-byte JPEG in 10 fragments, 5 272 064 samples, 2.059 s
of signal, 10 of 10 fragments recovered, image byte-identical, repeatable
across runs. Channel rate 18.475 kbit/s, below the 20 kbit/s symbol rate
because of framing and guards.

Three defects found by the verifier and fixed:

- the guard interval was only appended after the frame. Lab018 puts one on
  both sides, and the leading one is the one that matters: the first
  samples of a transmission are unreliable, so the preamble would have
  landed exactly on the transient. Invisible in software mode, which has
  no converter and no transient
- transmission was started before reception, contradicting section 8 of
  the specification and its own trap 3. The receiver now runs first and
  the first buffer is discarded, as in Lab024a
- packet and frame size were reported from the first frame only, while
  the tenth is 172 bytes against 536. Sizes are now per frame in the CSV
  and the report gives the honest range

Also fixed before verification: the JPEG did not fit its own 5000-byte
limit, so quality is now lowered until it does and reported as a result;
and the wall-clock rate was presented as a channel rate, which in software
mode measures computation, not a channel.

Artifacts are left out deliberately. data/processed/lab042 currently holds
the software reference; the committed results should be the real
experiment, which needs the cable.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

2026-08-11 13:28:47 +03:00

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