Audio formats and decoding

Audio is decoded entirely in FPGA fabric. There is no HPS-side decoder — the same as the video path.

Format	Decoded in core	Passthrough	Notes
AC-3 (Dolby Digital)	yes	yes	Every channel mode from 1.0 mono to 5.1, downmixed to stereo
MPEG-1 Layer II (MP2)	yes	no	Rare on DVD, universal on Video CD. 48/44.1/32 kHz
LPCM	yes	no	48 kHz stereo. 20/24-bit tracks play, truncated to 16-bit
DTS	no	yes	Passthrough to a receiver only

By default the core decodes to stereo and sends it over HDMI, which works on any display. For multichannel you want bitstream passthrough to an AV receiver.

AC-3

All channel modes are supported and downmixed to stereo for the HDMI output: mono (1.0), stereo (2.0), and the multichannel modes up to 5.1.

One deliberate exception

AC-3 1+1 dual mono (acmod 0) carries two independent programmes rather than one two-channel programme, so there is no correct way to mix them together. It is refused and plays silent. In a survey of 491 discs this appeared on 4 frames of 1 disc.

If a track plays silent and shows AUDIO UNSUPPORTED, cycle to another with B7.

MP2

MPEG-1 Layer II, at 32, 44.1 and 48 kHz.

It is a DVD-legal audio format and was used on some early PAL-region discs. It is included because the DVD specification permits it.

On Video CD and SVCD it is the opposite: MP2 is the only audio format those use, so every VCD depends on it.

MP2 has no passthrough encoding

In Passthru mode an MP2 track is silent on both outputs. Use Decode PCM for MP2 content.

The one gap is the MPEG-2 multichannel extension — a rare 5.1 variant. Its backwards-compatible stereo core should play, but no disc carrying one was available to verify, so such a track currently reports AUDIO UNSUPPORTED.

LPCM

Uncompressed, so there is nothing to decode. 48 kHz stereo.

DVD also permits 20-bit and 24-bit LPCM. Those tracks play, but the core takes the top 16 bits of each sample and discards the rest — the audio path out to HDMI is 16-bit, so the extra resolution has nowhere to go.

There is real fidelity loss on 20/24-bit tracks

Truncation, not rounding or dithering. On the sort of content that ships as high-bit-depth LPCM — concert recordings, audiophile music discs — this is the one place the core is audibly short of what the disc holds. A 16-bit LPCM track is unaffected and is exact.

96 kHz and multichannel LPCM are not supported. Multichannel is not a matter of effort: the DE10-Nano wires a single audio data line to its HDMI transmitter, which carries two channels, and the board routes no other pin for it. That is also why 5.1 has to leave as a compressed bitstream rather than as PCM.

DTS

There is no DTS decoder in the core. A DTS track is silent in Decode PCM mode.

To hear DTS, switch Audio Out to Passthru and send the bitstream to an AV receiver. Most DTS discs also carry an AC-3 track — cycling audio with B7 will usually find one that decodes.

Choosing a track

B7 cycles audio tracks, showing a popup with the track number and the language the disc declares — AUDIO 2/4 FR. The disc's own default is selected at start, influenced by the Player Language setting, the way a set-top player's setup screen works.

A disc's tracks are mapped through its own numbering, which can be sparse, so the numbers shown are the disc's rather than a simple count. See Controls.

Changing tracks inside a menu

Switching audio while a disc menu is open silences the menu's audio until you leave the menu. Menu audio otherwise plays normally on the default track.

Output level

Unreleased — AC-3 levels were wrong, and are corrected in the next release

Dolby Digital tracks were decoded 6 dB quieter than they should have been on stereo and mono soundtracks. 5.1 soundtracks were slightly loud for a separate reason, and the two faults partly cancelled — which is why the problem showed up as "stereo sounds weak" rather than as an obvious fault.

Both are fixed. After updating, expect stereo and mono Dolby Digital to be noticeably louder, and 5.1 to be a little quieter (about 1.5 dB). All of them now match what a set-top DVD player or VLC produces from the same disc, so levels are consistent between tracks and between discs. LPCM and MP2 were always correct and are unchanged.

DVD soundtracks are mastered a long way below full scale. Dialogue commonly sits 20 dB or more beneath the peaks so that loud scenes have headroom, which is why a film disc sounds quieter than a console core at the same TV volume — those emit chip audio near full scale almost continuously. The core reproduces what the disc holds rather than turning it up.

MiSTer's Core Volume control (in the MiSTer OSD's system menu, not this core's own OSD) can compensate. Press right past the top of the bar and it begins adding boost, displayed as + then ++. Each step is roughly +6 dB. It is a compressor rather than a plain gain: quiet material comes up while peaks are curved to land just under full scale, so pushing it does not clip. The level is stored per core, so setting it here does not affect anything else.

Unreleased

Boost requires MiSTer Main 20260603 or newer together with core support that arrives in the next release. On an older Main, or an older core, the boost steps do not appear and the control behaves as a plain attenuator.

Two limits worth knowing:

• It does not apply in Passthru. The core sends an untouched bitstream, so level is entirely your receiver's business — see Bitstream passthrough.
• It is a playback control, not a repair. It sits at the very end of the chain, after decoding and after the audio filter.

The same OSD change also enables MiSTer's audio filter for this core, which had never been advertised to the firmware and so never appeared.

A/V sync

Audio is locked to the video presentation timeline — the core builds a system clock referenced to what is actually on screen and paces audio against it, the way a real player slaves its audio to the recovered clock.

A/V Offset (Debug page) trims the relationship, defaulting to 0 ms. There should be no need to change it. Note that it binds at start and re-start events only — a mid-title change takes effect at the next seek or reload.

Silence checklist

If a disc plays with no sound:

1. Audio is On and Audio Out is Decode PCM — Passthru is silent on a display that cannot decode bitstreams, and on LPCM and MP2 tracks.
2. Try another track with B7 — the disc's default may be DTS, or a format the core cannot decode.
3. CSS ENCRYPTED on screen means audio is muted deliberately — see What you need.

If sound is present but simply too quiet, that is normal for DVD and is covered under Output level above.

Troubleshooting covers these in more detail.