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; 1+1 dual mono as left/right
MPEG-1 Layer II (MP2)	yes	no	Rare on DVD, universal on Video CD. 48/44.1/32 kHz
LPCM	yes	no	Every DVD-Video form: 48 or 96 kHz, 16/20/24-bit, 1–8 channels. Multichannel is downmixed to stereo; output is 16-bit
DTS	yes	yes	The DTS core, up to 5.1, downmixed to stereo
WAV (PCM file)	yes	no	16-bit stereo, 44.1/48 kHz — a plain audio file, not a disc

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.

1+1 dual mono carries two independent mono programmes rather than one stereo programme, for example the same dialogue in two languages. The first plays on the left and the second on the right, unmixed, so a listener can pick one with the TV's balance control. It is rare. In a survey of 491 discs it appeared on 4 frames of 1 disc.

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

WAV files¶

A .wav file selected from Load Video plays as audio with the bouncing logo on screen — see Loading a movie. WAV files are narrower than disc LPCM: 16-bit stereo only, at 44.1 or 48 kHz. Other shapes are refused with UNSUPPORTED IMAGE instead of being played as noise, and compressed formats (MP3, FLAC, AAC) have no decoder in the core.

44.1 kHz content is converted to the framework's fixed 48 kHz output by sample repetition — the pitch and duration are exact, but it is not a resampler. This is the same treatment Video CD audio has always had.

In Passthru a .wav plays as ordinary PCM on both outputs, because that is what it is. There is no bitstream to pass through, so the setting makes no difference to it.

CD audio images (`.cue`)¶

A ripped audio CD selected by its .cue sheet plays on the same PCM path, with track numbers and track skip like a disc in the drive — see Playing a .cue. CD audio is always 16-bit stereo at 44.1 kHz, so a sheet whose tracks are .wav files needs them in exactly that format; MP3 and FLAC sheets are refused. It needs the MiSTer_DVDcss add-on.

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 bitstream encoding, but it is not silent

There is no IEC 61937 bitstream format for MP2 in this core, so in Passthru an MP2 track is decoded and sent as ordinary PCM instead — the same audio Decode PCM produces. Nothing to change, and a VCD needs no trip back to Decode PCM. (Before v0.5.0 it really was silent.)

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. Every form DVD-Video allows plays:


Sample rate	48 or 96 kHz
Word length	16, 20 or 24 bit
Channels	1 to 8

The disc's bit rate caps the combinations (no more than two channels at 96 kHz/24-bit, for example), so the most demanding tracks are 96 kHz stereo and 48 kHz multichannel. Both are rare. They mostly turn up on audiophile "96/24" music discs and on some concert discs.

Multichannel LPCM is downmixed to stereo, the same way Dolby Digital and DTS are: the centre and surrounds are folded in at −3 dB and the LFE channel is left out. The board cannot send more than two PCM channels out (see below), so stereo is what reaches the TV. A disc does not label its LPCM channels, so the core assumes front left, front right, centre, LFE, then the surrounds for 5.1, the order FFmpeg-based players such as mpv and Kodi assume. Not every player agrees: VLC, for one, reads 5.1 LPCM in a different order.

96 kHz LPCM is converted to 48 kHz with a proper low-pass filter, which is also what most set-top players do. If you have set hdmi_audio_96k=1 in MiSTer.ini, the HDMI link runs at 96 kHz and a 96 kHz track plays at its own rate instead. The core reads that setting directly, so there is nothing to change in its own menu.

20-bit and 24-bit 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.

Why the output is stereo: 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. Decoding multichannel LPCM is not the problem; sending more than two PCM channels out is. That is also why 5.1 has to leave as a compressed bitstream rather than as PCM, and LPCM, which has no bitstream form, always leaves as the stereo downmix.

A track whose header holds a value DVD-Video does not allow (a 44.1 or 32 kHz rate, or an undefined word length) is muted and shows AUDIO UNSUPPORTED, rather than playing as noise.

DTS¶

DTS tracks decode in the core and play as stereo in Decode PCM, like AC-3. Every channel layout up to 5.1 is downmixed to stereo; the LFE channel is left out of the downmix, as other stereo players do.

The core decodes the DTS core stream, which is what DVD-Video carries: 48 kHz, at 768 or 1536 kbit/s. Extensions layered on top of it — DTS-ES 6.1 and 96/24 among them — are ignored, and the core underneath plays normally. Those extensions exist for a receiver to use, so for full 5.1 or 6.1, switch Audio Out to Passthru and send the bitstream to an AV receiver.

A DTS frame the core cannot decode is skipped rather than played as noise.

Choosing a track¶

B7 first shows the current audio track in a popup, with the track number and the language the disc declares — AUDIO 2/4 FR. Press it again while the popup is up to change to the next track. 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¶

Changed in v0.5.0 — Dolby Digital levels were wrong, and are corrected

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.

Requirements

Boost requires MiSTer Main 20260603 or newer together with core support added in v0.5.0. On an older Main, or a core older than that, 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 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:

Audio is On, and if the track is Dolby Digital or DTS, Audio Out is Decode PCM unless you have a receiver — Passthru sends those two as a bitstream, which an ordinary television cannot decode. LPCM and MP2 come out as PCM in either mode, so they are never silenced by this setting.
Try another track with B7 — the disc's default may be a format the core cannot decode.
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.