Bitstream passthrough¶

Set Audio Out = Passthru (SPDIF+HDMI) and the core stops decoding audio, sending the disc's undecoded AC-3 or DTS bitstream to an AV receiver to decode instead. This is what gets you real 5.1 rather than a stereo downmix, and it is the only way to hear DTS at all.

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.

What passes through and what does not¶

Format	In Passthru	Notes
AC-3 (Dolby Digital)	Bitstreamed	All channel modes
DTS	Bitstreamed	The only way to hear DTS — there is no DTS decoder in the core
LPCM	Silent	Use Decode PCM
MP2	Silent	Has no passthrough encoding

Passthru is not a better version of Decode

On a display that cannot decode AC-3 or DTS — an ordinary television, a monitor — Passthru is silent. Decode PCM works on anything. Only switch to Passthru if the audio is reaching a receiver that says it handles these formats.

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. This used to be visibly bad — several seconds of dropouts at every title start, and audible flapping on track changes — and is now fixed: titles lock 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 and MP2 are silent in Passthru; use Decode PCM for those discs.
Authored silence drops the receiver out of decode mode. Where a disc authors silence — a menu with no background audio, a gap between programmes — the receiver shows "Decoder Off" until audio returns, under a second later. There is no bitstream to send, and a receiver holds lock only on real data; a non-PCM hold and a pause burst were both tested and neither works. Many set-top players do the same.