Bitstream passthrough¶

Set Audio Out = Passthru (SPDIF+HDMI) and the disc's undecoded AC-3 or DTS bitstream goes to an AV receiver to decode instead of being decoded in the core. This is what gets you real 5.1 rather than a stereo downmix — for DTS it is also the only way to hear the extensions, such as DTS-ES 6.1, which the core's own decoder ignores. Tracks with no bitstream format — LPCM and MP2 — are still decoded and sent as ordinary PCM, so the setting never costs you sound.

The format is IEC 61937, the standard wrapper for carrying compressed audio inside what otherwise looks like an ordinary PCM stream. It is what a set-top DVD player's optical output does.

Two ways out¶

Optical S/PDIF — works with the stock MiSTer Main. The framework drives its S/PDIF signal to both the Digital I/O board's TOSLINK connector and the Analog I/O board's combo 3.5 mm mini-TOSLINK output, so either add-on board carries it.

Over HDMI — needs the MiSTer_DVDcss custom Main. IEC 61937 rides inside an ordinary 2-channel/48 kHz/16-bit stream at 1.536 Mbit/s, which is exactly what the DE10-Nano's single wired audio line to the HDMI transmitter carries. So 5.1 over HDMI needs no add-on board at all — just the custom Main and a receiver that advertises AC-3/DTS support in its EDID.

HDMI bitstream needs MiSTer_DVDcss

Optical S/PDIF passthrough works on the bare .rbf. Over HDMI it does not — see What you need. Nothing else about passthrough changes; the same Audio Out toggle drives both outputs.

Why HDMI needs the custom Main

Sending a bitstream to a device still expecting PCM produces full-scale noise. The HDMI transmitter's configuration is only reachable from the ARM side, so the core refuses to emit a bitstream over HDMI without an explicit acknowledgement that only MiSTer_DVDcss sets — after it has checked the display's EDID audio descriptors, which the stock Main does not parse at all. The safety is structural rather than a convention.

Over HDMI the transmitter is switched out of PCM mode only while a Dolby Digital or DTS track is actually playing, and switched back when the track changes, when you load another core, and when you reboot. You do not need to leave Passthru before loading something else.

If another core loses HDMI audio, update both halves and power-cycle

Earlier versions switched the transmitter as soon as Passthru was selected and never switched it back, so the next core was silent — and nothing but removing power cleared it. Update the core and MiSTer_DVDcss together: a current Main with an older core falls back to the older behaviour, because an older core cannot tell it which format is playing. See Troubleshooting.

What passes through and what does not¶

Format	In Passthru	Notes
AC-3 (Dolby Digital)	Bitstreamed	All channel modes
DTS	Bitstreamed	Including any extensions (DTS-ES, 96/24) the core's decoder ignores
LPCM	Sent as PCM	Decoded in the core and sent as ordinary stereo — see below
MP2	Sent as PCM	Same; this is what VCD and SVCD discs carry

The HDMI half needs a matching MiSTer_DVDcss

Over HDMI the wire format is set from the ARM side, so LPCM and MP2 come out as PCM only when the core and MiSTer_DVDcss are from the same release. Pairing a v0.5.0 core with an older Main leaves those two silent over HDMI — extract the release zip so both update together. Optical S/PDIF works with either.

New in v0.5.0 — LPCM and MP2 no longer go silent

These two used to be silent in Passthru, so a concert disc or a VCD needed a trip back to Decode PCM. They now come out as ordinary PCM, which is what a set-top player does with them. The switch is automatic and happens per track.

Passthru is no longer all-or-nothing. It sends whatever the disc's current audio track needs: a Dolby Digital or DTS track goes out as an undecoded bitstream for your receiver, and an LPCM or MP2 track is decoded in the core and goes out as ordinary PCM. Changing audio track with B7 switches the format on the wire, and your receiver will re-lock — a second or so of silence at the change is normal, and a real player does the same.

Passthru needs a receiver

In Passthru, Dolby Digital and DTS leave as bitstreams, which a plain television or monitor cannot decode. On such a display use Decode PCM, which decodes both in the core and sends stereo.

If the receiver names the format but plays static¶

Toggle SPDIF Byte Order (Normal / Swap). Payload byte order is the classic failure here — the receiver correctly identifies "Dolby Digital" from the stream's header but cannot make sense of the payload, so it hisses. It is a runtime toggle for exactly this reason.

Controlling the HDMI path from `MiSTer.ini`¶

By default the core engages the HDMI bitstream only when the display's EDID advertises AC-3/DTS support. That is the safe behaviour, but sinks do misreport — especially over ARC — so there is an override:

[DVD]
main=MiSTer_DVDcss

; HDMI bitstream for Audio Out = Passthru.
;   0 = auto  (default) engage only when the sink's EDID advertises AC-3/DTS
;   1 = off   never engage; HDMI behaves as it did before
;   2 = force engage regardless of EDID
dvd_hdmi_bitstream=0

Use 2 (force) when you know your receiver handles Dolby Digital or DTS but the core is not engaging — a receiver reached over ARC often does not advertise its capabilities correctly. Use 1 (off) to rule the HDMI path out entirely while diagnosing something else.

Finding out what it decided

The custom Main writes /tmp/dvd_hdmi_audio.log with the stage-by-stage result — what the EDID said, whether the bitstream path engaged, and why not if it did not. That is the first thing to read when HDMI passthrough is silent.

Track changes and startup¶

The receiver needs a moment to lock onto the bitstream, and switching audio tracks re-establishes it. A title locks within a couple of seconds and track changes are near-instant.

Limitations¶

Core DTS only — 48 kHz, up to 16-bit. No DTS-HD, no 96 kHz, no high-bit-depth variants. DVDs do not carry those.
LPCM always leaves as 16-bit stereo PCM. IEC 61937 has no LPCM form, so multichannel LPCM is decoded and downmixed to stereo, and 20/24-bit is truncated to 16, in both Audio Out modes. See Audio formats.