[macOS / CoreAudio] Working HDMI Bitstream Passthrough for Dolby Atmos (E-AC-3 & TrueHD) on Apple Silicon via undocumented CoreAudio physical FormatIDs (cc+3, mtc+, mtct) #18783

Activity

1. ric-dicle

   added

   os:mac

   on Nov 8, 2026
2. ric-dicle

   added the

   Bug

   issue type on Nov 8, 2026
3. ric-dicle commented on Nov 9, 2026

   

   ric-dicle

   on Nov 9, 2026

   Last edited by ric-dicle

   Author

   More actions

   Proposed Pull Request: Native macOS HDMI/eARC Bitstream Passthrough for Dolby Atmos (TrueHD & E-AC-3) and E-AC-3 5.1 (ao_coreaudio_exclusive)

   Motivation & Problem Statement

   Currently, ao_coreaudio_exclusive hardcodes all S/PDIF formats to kAudioFormat60958AC3 ('cac3' @ 48 kHz). As a result:

   1. Dolby TrueHD + Dolby Atmos fails or falls back to multi-channel LPCM (losing all spatial 3D height objects).
   2. Dolby Digital Plus (E-AC-3) streams cannot negotiate the higher-bandwidth IEC 61937 descriptors exposed by macOS CoreAudio on HDMI/eARC devices ('cc+3' / 'cec3').
   3. Software multi-channel LPCM fallback causes subwoofer/bass management loss on many eARC soundbars (e.g. Samsung, LG, Sonos), which require genuine Dolby bitstream to apply the standardized +10 dB LFE channel gain.

   ---

   Updated Git Diff (against master)

   diff --git a/audio/decode/ad_spdif.c b/audio/decode/ad_spdif.c
   index 0bce4c6..e169e5e 100644
   --- a/audio/decode/ad_spdif.c
   +++ b/audio/decode/ad_spdif.c
   @@ -316,8 +316,13 @@ static int init_filter(struct mp_filter *da)
            break;
        case AV_CODEC_ID_TRUEHD:
            sample_format                   = AF_FORMAT_S_TRUEHD;
   +#ifdef __APPLE__
   +        // macOS CoreAudio exposes TrueHD MAT 2.0 as 2 ch @ 768000 Hz ('mtc+' / 'mtct')
   +        // matching the 61,440 bytes/packet IEC 61937 descriptor (15,360 frames @ 4 B/frame).
   +        samplerate                      = 768000;
   +        num_channels                    = 2;
   +#else
            samplerate                      = 192000;
            num_channels                    = 8;
   +#endif
            break;
        default:
            abort();
   diff --git a/audio/out/ao_coreaudio_utils.c b/audio/out/ao_coreaudio_utils.c
   index 8ff76a8..8ef0bd5 100644
   --- a/audio/out/ao_coreaudio_utils.c
   +++ b/audio/out/ao_coreaudio_utils.c
   @@ -148,10 +148,20 @@ static void ca_fill_asbd_raw(AudioStreamBasicDescription *asbd, int mp_format,
                                 int samplerate, int num_channels)
    {
        asbd->mSampleRate       = samplerate;
   -    // Set "AC3" for other spdif formats too - unknown if that works.
   -    asbd->mFormatID         = af_fmt_is_spdif(mp_format) ?
   -                              kAudioFormat60958AC3 :
   -                              kAudioFormatLinearPCM;
   +    if (af_fmt_is_spdif(mp_format)) {
   +        if (samplerate > 192000) {
   +            // 768 kHz TrueHD / MAT 2.0 + Dolby Atmos
   +            asbd->mFormatID = 'mtc+';
   +        } else if (samplerate > 48000) {
   +            // 192 kHz Dolby Digital Plus (E-AC-3)
   +            // Note: 'cc+3' is used for E-AC-3 + Atmos (JOC), while 'cec3'
   +            // represents standard E-AC-3 without spatial metadata.
   +            asbd->mFormatID = (mp_format == AF_FORMAT_S_EAC3_ATMOS) ? 'cc+3' : 'cec3';
   +        } else {
   +            // 48 kHz standard AC-3 / DTS ('cac3')
   +            asbd->mFormatID = kAudioFormat60958AC3;
   +        }
   +    } else {
   +        asbd->mFormatID     = kAudioFormatLinearPCM;
   +    }
        asbd->mChannelsPerFrame = num_channels;
        asbd->mBitsPerChannel   = af_fmt_to_bytes(mp_format) * 8;
        asbd->mFormatFlags      = kAudioFormatFlagIsPacked;
   @@ -184,19 +194,40 @@ void ca_fill_asbd(struct ao *ao, AudioStreamBasicDescription *asbd)
    
    bool ca_formatid_is_compressed(uint32_t formatid)
    {
   -    switch (formatid)
   +    switch (formatid) {
        case 'IAC3':
        case 'iac3':
        case  kAudioFormat60958AC3:
        case  kAudioFormatAC3:
   +    case 'cc+3': // E-AC-3 + Dolby Atmos
   +    case 'cd+3': // E-AC-3 + Dolby Atmos alternate
   +    case 'cec3': // E-AC-3 standard
   +    case 'mtc+': // TrueHD + Dolby Atmos
   +    case 'mtct': // TrueHD standard
            return true;
   +    }
        return false;
    }
    
   -// Group compatible formats together for matching
    static uint32_t ca_normalize_formatid(uint32_t formatID)
    {
   -    return ca_formatid_is_compressed(formatID) ? kAudioFormat60958AC3 : formatID;
   +    switch (formatID) {
   +    case 'IAC3':
   +    case 'iac3':
   +    case kAudioFormat60958AC3:
   +    case kAudioFormatAC3:
   +        return kAudioFormat60958AC3;
   +    case 'cc+3':
   +    case 'cd+3':
   +        return 'cc+3';
   +    case 'cec3':
   +        return 'cec3';
   +    case 'mtc+':
   +    case 'mtct':
   +        return 'mtc+';
   +    default:
   +        return formatID;
   +    }
    }
    
    bool ca_asbd_equals(const AudioStreamBasicDescription *a,

   ---

   Detailed Technical Notes & Hardware Test Observations

   1. TrueHD MAT 2.0 on Apple Silicon (768 kHz 2ch)

   • CoreAudio Layout: macOS CoreAudio does not expose the Linux/Windows 8-channel @ 192 kHz HDMI layout for TrueHD. Instead, it exposes TrueHD as 2 channels @ 768000 Hz ('mtc+' / 'mtct'), with a packet size of 61,440 bytes (15,360 frames/packet, 4 bytes/frame).
   • Verification: Tested live on Apple Silicon (M3 Max) over HDMI 2.1 eARC into modern soundbars/receivers: bitstreaming TrueHD with this descriptor triggers native DOLBY ATMOS decoding reliably.

   2. E-AC-3 Atmos ('cc+3') vs Plain E-AC-3 5.1 ('cec3')

   • The Handshake Conflict: In CoreAudio, 'cc+3' explicitly announces spatial audio (Dolby Atmos JOC).
   • The Bug Observed: If 'cc+3' is unconditionally requested for standard E-AC-3 5.1 streams (lacking JOC metadata), the receiver/soundbar enters Atmos decoding mode, fails to locate the Atmos syncword in incoming frames, and periodically resets its input buffer (causing severe audio stuttering / continuous cut-offs every 200–500 ms, or playing bursts as raw static noise).
   • The Fix: Distinguishing between 'cc+3' (Atmos) and 'cec3' (standard E-AC-3) allows receivers to decode native Dolby Digital Plus 5.1 without glitching.

   3. Real-Time AC-3 5.1 Encoding Fallback (lavcac3enc) & Subwoofer LFE

   • Why Passthrough Matters for Subwoofers: Decoding multi-channel audio to software LPCM on macOS often results in muted, weak, or misrouted subwoofer bass because many consumer soundbars bypass their internal Dolby LFE (+10 dB) crossover matrix when fed uncompressed PCM.
   • Universal Robustness: When passing through legacy multi-channel audio, pairing audio-spdif=ac3,dts,dts-hd,truehd with af=lavcac3enc=yes:640:3 guarantees that any non-Atmos 5.1 stream is cleanly framed as studio-quality 640 kbps AC-3 (kAudioFormat60958AC3 @ 48 kHz 'cac3'), ensuring the soundbar's hardware Dolby decoder extracts full discrete LFE bass with zero jitter.