RDPSND — Audio Output

API reference for the FreeRDP audio output virtual channel (RDPSND / MS-RDPEA), covering server and client contexts, format negotiation, and audio data flow.

The RDPSND channel implements [MS-RDPEA] — Remote Desktop Protocol: Audio Output Virtual Channel Extension. It streams audio from the server to the client.

SVC channel name: rdpsnd (RDPSND_CHANNEL_NAME)
DVC channel names: AUDIO_PLAYBACK_DVC, AUDIO_PLAYBACK_LOSSY_DVC

Server header: <freerdp/server/rdpsnd.h>
Client header: <freerdp/client/rdpsnd.h>
Common types: <freerdp/channels/rdpsnd.h> (re-exports <freerdp/codec/audio.h>)

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Server API

RdpsndServerContext lifecycle

RdpsndServerContext* rdpsnd_server_context_new(HANDLE vcm);
void                 rdpsnd_server_context_free(RdpsndServerContext* context);
void                 rdpsnd_server_context_reset(RdpsndServerContext* context);

vcm is the virtual channel manager handle. reset reinitialises the context without freeing, for reuse after a session reconnect.

Helper functions

HANDLE rdpsnd_server_get_event_handle(RdpsndServerContext* context);
UINT   rdpsnd_server_handle_messages(RdpsndServerContext* context);

Use rdpsnd_server_get_event_handle() in a wait-loop and call rdpsnd_server_handle_messages() when signalled to process incoming client PDUs.

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RdpsndServerContext fields

data · void*

Server-defined private pointer.

use_dynamic_virtual_channel · BOOL

Set to TRUE before Initialize to open via DVC (AUDIO_PLAYBACK_DVC) instead of the legacy SVC.

server_formats · AUDIO_FORMAT*

Array of AUDIO_FORMAT structs the server supports. Set before calling Initialize.

num_server_formats · size_t

Length of server_formats.

src_format · AUDIO_FORMAT*

The PCM format of the audio source (sample rate, channels, bits-per-sample). Set before SendSamples.

latency · UINT32

Requested audio buffer / latency in milliseconds.

client_formats · AUDIO_FORMAT*

Populated by the channel after format negotiation with the client. Read-only for the server.

num_client_formats · UINT16

Number of entries in client_formats.

selected_client_format · UINT16

Index into client_formats of the currently active format. Set by SelectFormat.

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Server API calls

Initialize · psRdpsndServerInitialize

typedef UINT (*psRdpsndServerInitialize)(RdpsndServerContext* context, BOOL ownThread);

Opens the channel and begins format negotiation. If ownThread = TRUE the implementation spawns a worker thread; otherwise the caller must drive messages via rdpsnd_server_handle_messages().

SelectFormat · psRdpsndServerSelectFormat

typedef UINT (*psRdpsndServerSelectFormat)(RdpsndServerContext* context,
                                          UINT16 client_format_index);

Selects the client audio format at client_format_index (index into client_formats). Must be called before SendSamples.

SendFormats · psRdpsndServerSendFormats

typedef UINT (*psRdpsndServerSendFormats)(RdpsndServerContext* context);

Re-sends the server format list and version PDU. Normally called automatically by Initialize; call manually to restart the protocol after Close.

SendSamples · psRdpsndServerSendSamples

typedef UINT (*psRdpsndServerSendSamples)(RdpsndServerContext* context,
                                         const void* buf, size_t nframes,
                                         UINT16 wTimestamp);

Sends PCM audio data in src_format. nframes is the number of audio frames (not bytes). The channel converts to the negotiated client format internally if a DSP is configured.

SendSamples2 · psRdpsndServerSendSamples2

typedef UINT (*psRdpsndServerSendSamples2)(RdpsndServerContext* context,
                                          UINT16 formatNo, const void* buf,
                                          size_t size, UINT16 timestamp,
                                          UINT32 audioTimeStamp);

Sends already-encoded audio via a Wave2 PDU. formatNo is an index into client_formats. Bypasses any internal DSP conversion.

SetVolume · psRdpsndServerSetVolume

typedef UINT (*psRdpsndServerSetVolume)(RdpsndServerContext* context,
                                       UINT16 left, UINT16 right);

Sets the client-side playback volume. Valid range is 0x0000 (mute) to 0xFFFF (full).

Training · psRdpsndServerTraining

typedef UINT (*psRdpsndServerTraining)(RdpsndServerContext* context,
                                      UINT16 timestamp, UINT16 packsize,
                                      BYTE* data);

Sends a Training PDU used for round-trip time measurement.

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Server callbacks

Activated · psRdpsndServerActivated

typedef void (*psRdpsndServerActivated)(RdpsndServerContext* context);

Called (from the worker thread) when the client has completed format negotiation and is ready to receive audio. Start streaming from this callback.

TrainingConfirm · psRdpsndServerTrainingConfirm

typedef UINT (*psRdpsndServerTrainingConfirm)(RdpsndServerContext* context,
                                             UINT16 timestamp, UINT16 packsize);

Called when the client confirms a Training PDU. Use to calculate round-trip latency.

ConfirmBlock · psRdpsndServerConfirmBlock

typedef UINT (*psRdpsndServerConfirmBlock)(RdpsndServerContext* context,
                                          BYTE confirmBlockNum, UINT16 wtimestamp);

Called when the client acknowledges a WaveConfirm PDU. Used for flow control.

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Client Device Plugin

The client side uses an rdpsndDevicePlugin (audio output backend — e.g. PulseAudio, ALSA, Windows CoreAudio).

struct rdpsnd_device_plugin {
    rdpsndPlugin*           rdpsnd;
    pcFormatSupported       FormatSupported;   /* BOOL(device, format) */
    pcOpen                  Open;              /* BOOL(device, format, latency) */
    pcGetVolume             GetVolume;         /* UINT32(device) */
    pcSetVolume             SetVolume;         /* BOOL(device, value) */
    pcPlay                  Play;              /* UINT(device, data, size) */
    pcPlayEx                PlayEx;            /* UINT(device, format, data, size) */
    pcClose                 Close;
    pcFree                  Free;
    pcDefaultFormat         DefaultFormat;     /* negotiate preferred format */
    pcServerFormatAnnounce  ServerFormatAnnounce; /* called with server formats */
};

Register a custom backend by implementing freerdp_rdpsnd_client_subsystem_entry (RDPSND_DEVICE_EXPORT_FUNC_NAME) returning a populated rdpsndDevicePlugin.

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Audio Codec Support

GSM 6.10

Always available — built-in codec in WinPR. No extra dependencies.

MP3 / PCM

MP3 encoding support requires FFmpeg (-DWITH_FFMPEG=ON). Raw PCM is always available.

OPUS

Requires libopus. Enable with -DWITH_OPUS=ON.

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Minimal Server Usage

#include <freerdp/server/rdpsnd.h>

static void on_rdpsnd_activated(RdpsndServerContext* context)
{
    /* Pick first mutually supported format */
    context->SelectFormat(context, 0);
    /* Feed audio from your source... */
}

RdpsndServerContext* rdpsnd = rdpsnd_server_context_new(vcm);
rdpsnd->data      = my_state;
rdpsnd->Activated = on_rdpsnd_activated;

/* Define PCM source format */
AUDIO_FORMAT fmt = { WAVE_FORMAT_PCM, 2, 44100, 176400, 4, 16, 0, NULL };
rdpsnd->src_format = &fmt;

/* Advertise formats */
rdpsnd->server_formats     = my_formats;
rdpsnd->num_server_formats = my_format_count;

rdpsnd->Initialize(rdpsnd, TRUE); /* own thread */