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digirta

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This is a RTA meter for Digitakt I

digirta is custom firmware for the Elektron Digitakt, by @Alez22. Open source under MIT, 1 star on GitHub.

forElektron Digitakt·by@Alez22

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RTA for Digitakt

An octave-band real-time analyzer for the original Digitakt (Mk1), OS 1.53, as an elekloader mod. It shows the frequency balance of the main outputs or of the inputs as a popup over any page.

USE AT YOUR OWN RISK. This modifies your instrument's firmware. Back up your projects and sounds, and keep a stock OS 1.53 file for recovery.

Use

KeysWhat they do
FUNC + DOWN, tapOpens the analyzer; it stays open
FUNC + DOWN, holdShows the analyzer until you let go, for a quick check
FUNC + DOWN or NOCloses it
LEFT / RIGHTWhile it is open: choose the source
  • Sources: OUT L, OUT R, OUT L+R (the main outputs, after effects, master compressor and level) and IN L, IN R, IN L+R.
  • 10 bars, one per octave, from 31 Hz to 16 kHz.
    • The scale runs from 0 dB (a full-scale sine) down to −60 dB, with a dotted line every 12 dB.
    • Bars rise at once and fall at 20 dB/s. A peak mark holds for 1.5 s.
  • CPU nn in the title is the analysis' share of the UI's time.
    • On busy projects (many Sophie or STRING tracks) it can reach the top.
    • The audio is never affected; only the UI and the bars slow down.
  • While the analyzer is closed it costs nothing.

How it works

  • Events only. It patches no firmware bytes: it uses only elekloader core's events (render entry and exit, keys, UI tick, draw). It can be installed with Sophie, STRING, digihealth and other mods.
  • Tap (tap.c):
    • At render exit, the audio interrupt copies the block just computed into an 8 KB ring, as mono int16. The block comes from the DMA buffer that feeds the outputs, or from the one filled by the inputs.
    • The copy takes 32 samples per block and runs only while the analyzer is open.
  • Analysis (rta.c), in the UI task:
    • High bands, 500 Hz – 16 kHz: a 1024-point FFT at 48 kHz.
    • Low bands, 31 – 250 Hz: a 512-point FFT of a 3 kHz stream, made by a CIC decimator.
    • Both use a Hann window, fixed-point maths and no EMAC.
    • Each band is the sum of its bins' power.
    • At most one FFT per UI tick, so the work per tick stays bounded.
  • UI (ui.c): keys, the popup, bars and peaks.

The design and its history are in SPEC.md.

Build (developers)

You need the ColdFire cross toolchain, Python 3 (standard library only), elekloader and your own stock OS 1.53 .syx.

make test          # host tests: FFT against a reference DFT, band levels, noise, ballistics
make cross-check   # compile for the Digitakt's ColdFire

From the elekloader checkout:

STOCK=/path/to/Digitakt_OS1.53.syx
python3 -m elekloader.sdk.build mods/core --stock $STOCK
python3 -m elekloader.sdk.build /path/to/digirta --stock $STOCK
python3 -m elekloader.patch --stock $STOCK \
    --mod mods/core/out/core-2.1.elemod \
    --mod /path/to/digirta/out/digirta-0.3.0.elemod \
    --out Digitakt_OS1.53_RTA.syx --version R003

python3 scripts/gen_tables.py > rta_tables.inc regenerates the window, twiddle and band tables.

All addresses are for Digitakt Mk1 OS 1.53 only.

License

MIT, see LICENSE. Independent of, and not endorsed by, Elektron; it contains no Elektron firmware.