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// Copyright 2017 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
/// Configuration for a capturer which will receive a loopback stream
/// a system output.
table LoopbackAudioCapturerConfiguration {
/// Configuration for a capturer which will receive a stream from an
/// input device.
table InputAudioCapturerConfiguration {
1: AudioCaptureUsage usage;
/// Configuration for an audio Capturer.
union AudioCapturerConfiguration {
1: LoopbackAudioCapturerConfiguration loopback;
2: InputAudioCapturerConfiguration input;
// TODO(mpuryear): Routing policy needs to become more capable than this.
// Clients will need to be able to request sets of inputs/outputs/renderers,
// make changes to theses sets, have their requests vetted by policy (do they
// have the permission to capture this private stream, do they have the
// permission to capture at this frame rate, etc...). Eventually, this
// functionality will need to be expressed at the AudioPolicy level, not here.
// TODO(mpuryear): can we come up with better names than these? Both are
// really async modes under the hood).
// TODO(mpuryear): Add a new method to replace `DiscardAllPackets`, which will
// be removed from `StreamSource`.
// TODO(mpuryear): Fix obsolete docs.
// TODO(mpuryear): Specify the way in which timestamps relative to a different
// clock (such as an audio domain clock) may be delivered to a client.
/// AudioCapturer
/// An AudioCapturer is an interface returned from an's
/// CreateAudioCapturer method, which may be used by clients to capture audio
/// from either the current default audio input device, or the current default
/// audio output device depending on the flags passed during creation.
/// ** Format support **
/// See (Get|Set)StreamType below. By default, the captured stream type will be
/// initially determined by the currently configured stream type of the source
/// that the AudioCapturer was bound to at creation time. Users may either fetch
/// this type using GetStreamType, or they may choose to have the media
/// resampled or converted to a type of their choosing by calling SetStreamType.
/// Note: the stream type may only be set while the system is not running,
/// meaning that there are no pending capture regions (specified using CaptureAt)
/// and that the system is not currently running in 'async' capture mode.
/// ** Buffers and memory management **
/// Audio data is captured into a shared memory buffer (a VMO) supplied by the
/// user to the AudioCapturer during the AddPayloadBuffer call. Please note the
/// following requirements related to the management of the payload buffer.
/// ++ The payload buffer must be supplied before any capture operation may
/// start. Any attempt to start capture (via either CaptureAt or
/// StartAsyncCapture) before a payload buffer has been established is an
/// error.
/// ++ The payload buffer may not be changed while there are any capture
/// operations pending.
/// ++ The stream type may not be changed after the payload buffer has been set.
/// ++ The payload buffer must be an integral number of audio frame sizes (in
/// bytes)
/// ++ When running in 'async' mode (see below), the payload buffer must be at
/// least as large as twice the frames_per_packet size specified during
/// StartAsyncCapture.
/// ++ The handle to the payload buffer supplied by the user must be readable,
/// writable, mappable and transferable.
/// ++ Users should always treat the payload buffer as read-only.
/// ** Synchronous vs. Asynchronous capture mode **
/// The AudioCapturer interface can be used in one of two mutually exclusive
/// modes: Synchronous and Asynchronous. A description of each mode and their
/// tradeoffs is given below.
/// ** Synchronous mode **
/// By default, AudioCapturer instances are running in 'sync' mode. They will
/// only capture data when a user supplies at least one region to capture into
/// using the CaptureAt method. Regions supplied in this way will be filled in
/// the order that they are received and returned to the client as StreamPackets
/// via the return value of the CaptureAt method. If an AudioCapturer instance
/// has data to capture, but no place to put it (because there are no more
/// pending regions to fill), the next payload generated will indicate that their
/// has been an overflow by setting the Discontinuity flag on the next produced
/// StreamPacket. Synchronous mode may not be used in conjunction with
/// Asynchronous mode. It is an error to attempt to call StartAsyncCapture while
/// the system still regions supplied by CaptureAt waiting to be filled.
/// If a user has supplied regions to be filled by the AudioCapturer instance in
/// the past, but wishes to reclaim those regions, they may do so using the
/// DiscardAllPackets method. Calling the DiscardAllPackets method will cause
/// all pending regions to be returned, but with `NO_TIMESTAMP` as their
/// StreamPacket's PTS. See "Timing and Overflows", below, for a discussion of
/// timestamps and discontinuity flags. After a DiscardAllPackets operation,
/// an OnEndOfStream event will be produced. While an AudioCapturer will never
/// overwrite any region of the payload buffer after a completed region is
/// returned, it may overwrite the unfilled portions of a partially filled
/// buffer which has been returned as a result of a DiscardAllPackets operation.
/// ** Asynchronous mode **
/// While running in 'async' mode, clients do not need to explicitly supply
/// shared buffer regions to be filled by the AudioCapturer instance. Instead, a
/// client enters into 'async' mode by calling StartAsyncCapture and supplying a
/// callback interface and the number of frames to capture per-callback. Once
/// running in async mode, the AudioCapturer instance will identify which
/// payload buffer regions to capture into, capture the specified number of
/// frames, then deliver those frames as StreamPackets using the OnPacketCapture
/// FIDL event. Users may stop capturing and return the AudioCapturer instance to
/// 'sync' mode using the StopAsyncCapture method.
/// It is considered an error to attempt any of the following operations.
/// ++ To attempt to enter 'async' capture mode when no payload buffer has been
/// established.
/// ++ To specify a number of frames to capture per payload which does not permit
/// at least two contiguous capture payloads to exist in the established
/// shared payload buffer simultaneously.
/// ++ To send a region to capture into using the CaptureAt method while the
/// AudioCapturer instance is running in 'async' mode.
/// ++ To attempt to call DiscardAllPackets while the AudioCapturer instance is
/// running in 'async' mode.
/// ++ To attempt to re-start 'async' mode capturing without having first
/// stopped.
/// ++ To attempt any operation except for SetGain while in the process of
/// stopping.
/// ** Synchronizing with a StopAsyncCapture operation **
/// Stopping asynchronous capture mode and returning to synchronous capture mode
/// is an operation which takes time. Aside from SetGain, users may not call any
/// other methods on the AudioCapturer interface after calling StopAsyncCapture
/// (including calling StopAsyncCapture again) until after the stop operation has
/// completed. Because of this, it is important for users to be able to
/// synchronize with the stop operation. Two mechanisms are provided for doing
/// so.
/// The first is to use StopAsyncCapture (not the NoReply variant). When the user's
/// callback has been called, they can be certain that stop operation is complete
/// and that the AudioCapturer instance has returned to synchronous operation
/// mode.
/// The second way to determine that a stop operation has completed is to use the
/// flags on the packets which get delivered via the user-supplied
/// AudioCapturerCallback interface after calling StopAsyncCapture. When
/// asked to stop, any partially filled packet will be returned to the user, and
/// the final packet returned will always have the end-of-stream flag (kFlagsEos)
/// set on it to indicate that this is the final frame in the sequence. If
/// there is no partially filled packet to return, the AudioCapturer will
/// synthesize an empty packet with no timestamp, and offset/length set to zero,
/// in order to deliver a packet with the end-of-stream flag set on it. Once
/// users have seen the end-of-stream flag after calling stop, the AudioCapturer
/// has finished the stop operation and returned to synchronous operating mode.
/// ** Timing and Overflows **
/// All media packets produced by an AudioCapturer instance will have their PTS
/// field filled out with the capture time of the audio expressed as a timestamp
/// given by the reference clock timeline. Note: this timestamp is actually a
/// capture timestamp, not a presentation timestamp (it is more of a CTS than a
/// PTS) and is meant to represent the underlying system's best estimate of the
/// capture time of the first frame of audio, including all outboard and hardware
/// introduced buffering delay. As a result, all timestamps produced by an
/// AudioCapturer should be expected to be in the past relative to 'now' on the
/// stream's reference clock timeline.
/// The one exception to the "everything has an explicit timestamp" rule is when
/// discarding submitted regions while operating in synchronous mode. Discarded
/// packets have no data in them, but FIDL demands that all pending
/// method-return-value callbacks be executed. Because of this, the regions will
/// be returned to the user, but their timestamps will be set to
/// `NO_TIMESTAMP`, and their payload sizes will be set to zero. Any
/// partially filled payload will have a valid timestamp, but a payload size
/// smaller than originally requested. The final discarded payload (if there
/// were any to discard) will be followed by an OnEndOfStream event.
/// Two StreamPackets delivered by an AudioCapturer instance are 'continuous' if
/// the first frame of audio contained in the second packet was capture exactly
/// one nominal frame time after the final frame of audio in the first packet.
/// If this relationship does not hold, the second StreamPacket will have the
/// 'kFlagDiscontinuous' flag set in it's flags field.
/// Even though explicit timestamps are provided on every StreamPacket produced,
/// users who have very precise timing requirements are encouraged to always
/// reason about time by counting frames delivered since the last discontinuity,
/// rather than simply using the raw capture timestamps. This is because the
/// explicit timestamps written on continuous packets may have a small amount of
/// rounding error based on whether or not the units of the capture timeline
/// reference clock are divisible by the chosen audio frame rate.
/// Users should always expect the first StreamPacket produced by an
/// AudioCapturer to have the discontinuous flag set on it (as there is no
/// previous packet to be continuous with). Similarly, the first StreamPacket
/// after a DiscardAllPackets or a Stop/Start cycle will always be
/// discontinuous. After that, there are only two reasons that a StreamPacket
/// will ever be discontinuous:
/// 1) The user is operating an synchronous mode and does not supply regions to
/// be filled quickly enough. If the next continuous frame of data has not
/// been captured by the time it needs to be purged from the source buffers,
/// an overflow has occurred and the AudioCapturer will flag the next captured
/// region as discontinuous.
/// 2) The user is operating in asynchronous mode and some internal error
/// prevents the AudioCapturer instance from capturing the next frame of audio
/// in a continuous fashion. This might be high system load or a hardware
/// error, but in general it is something which should never normally happen.
/// In practice, however, if it does, the next produced packet will be flagged
/// as being discontinuous.
/// ** Synchronous vs. Asynchronous Trade-offs **
/// The choice of operating in synchronous vs. asynchronous mode is up to the
/// user, and depending on the user's requirements, there are some advantages and
/// disadvantages to each choice.
/// Synchronous mode requires only a single Zircon channel under the hood and can
/// achieve some small savings because of this. In addition, the user has
/// complete control over the buffer management. Users specify exactly where
/// audio will be captured to and in what order. Because of this, if users do
/// not need to always be capturing, it is simple to stop and restart the capture
/// later (just by ceasing to supply packets, then resuming later on). Payloads
/// do not need to be uniform in size either, clients may specify payloads of
/// whatever granularity is appropriate.
/// The primary downside of operating in synchronous mode is that two messages
/// will need to be sent for every packet to be captured. One to inform the
/// AudioCapturer of the instance to capture into, and one to inform the user
/// that the packet has been captured. This may end up increasing overhead and
/// potentially complicating client designs.
/// Asynchronous mode has the advantage requiring only 1/2 of the messages,
/// however, when operating in 'async' mode, AudioCapturer instances have no way
/// of knowing if a user is processing the StreamPackets being sent in a timely
/// fashion, and no way of automatically detecting an overflow condition. Users
/// of 'async' mode should be careful to use a buffer large enough to ensure that
/// they will be able to process their data before an AudioCapturer will be
/// forced to overwrite it.
// ** Future Directions (aka TODOs) **
// ++ Consider adding a 'zero message' capture mode where the AudioCapturer
// simply supplies a linear transformation and some buffer parameters (max
// audio hold time) each time that it is started in 'async' mode, or each
// time an internal overflow occurs in 'async' mode. Based on this
// information, client should know where the capture write pointer is at all
// times as a function of the transformation removing the need to send any
// buffer position messages. This would reduce the operational overhead just
// about as low as it could go, and could allow for the lowest possible
// latency for capture clients. OTOH - it might be better to achieve this
// simply by allowing clients to be granted direct, exclusive access to the
// driver level of capture if no resampling, reformatting, or sharing is
// needed.
// ++ Consider providing some mechanism by which users may specify the exact
// time at which they want to capture data.
// ++ Allow for more complex routing/mixing/AEC scenarios and place this under
// the control of the policy manager.
// ++ Define and enforce access permissions and downsampling requirements for
// sensitive content. Enforce using the policy manager.
// ++ Consider allowing the mixer to produce compressed audio.
protocol AudioCapturer {
compose StreamBufferSet;
compose StreamSource;
/// Sets the stream type of the stream to be delivered. Causes the source
/// material to be reformatted/resampled if needed in order to produce the
/// requested stream type. Note that the stream type may not be changed
/// after the payload buffer has been established.
SetPcmStreamType(AudioStreamType stream_type);
/// Explicitly specify a region of the shared payload buffer for the audio
/// input to capture into.
CaptureAt(uint32 payload_buffer_id, uint32 payload_offset,
uint32 frames) -> (StreamPacket captured_packet);
/// Place the AudioCapturer into 'async' capture mode and begin to produce
/// packets of exactly 'frames_per_packet' number of frames each. The
/// OnPacketProduced event (of StreamSink) will be used to inform the client
/// of produced packets.
StartAsyncCapture(uint32 frames_per_packet);
/// Stop capturing in 'async' capture mode and (optionally) deliver a
/// callback that may be used by the client if explicit synchronization
/// is needed.
StopAsyncCapture() -> ();
/// Binds to the gain control for this AudioCapturer.
BindGainControl(request<> gain_control_request);
/// Retrieves the stream's reference clock. The returned handle will have READ, DUPLICATE
/// and TRANSFER rights, and will refer to a zx::clock that is MONOTONIC and CONTINUOUS.
GetReferenceClock() -> (handle<clock> reference_clock);
/// Sets the reference clock that controls this capturer's playback rate. If the input
/// parameter is valid, it must have READ, DUPLICATE, TRANSFER rights and refer to a clock
/// that is MONOTONIC and CONTINUOUS. If instead ZX_HANDLE_INVALID is passed, then this
/// capturer will use the so-called 'optimal' clock generated by AudioCore for this stream.
/// Calling |SetReferenceClock| during Async capture will generally cause a capture
/// discontinuity, with audio dropped or silence inserted (based on the discontinuity).
/// Cases that PRESERVE continuity would include setting as reference either the 'optimal'
/// clock provided by AudioCore, or the clone of a client-controlled reference clock.
SetReferenceClock(handle<clock>? reference_clock);
/// Sets the usage of the capture stream. This may be changed on the fly, but
/// packets in flight may be affected by the new usage. By default the
/// Capturer is created with the FOREGROUND usage.
SetUsage(AudioCaptureUsage usage);
// ////////////////////////////////////////////////////////////////////////
// StreamBufferSet methods
// See stream.fidl.
// ////////////////////////////////////////////////////////////////////////
// StreamSource methods
// See stream.fidl.
// ////////////////////////////////////////////////////////////////////////
// Methods to be deprecated
// These methods will go away.
/// Gets the currently configured stream type. Note: for an AudioCapturer
/// which was just created and has not yet had its stream type explicitly
/// set, this will retrieve the stream type -- at the time the AudioCapturer
/// was created -- of the source (input or looped-back output) to which the
/// AudioCapturer is bound.
// TODO(mpuryear): Get rid of this. Eventually, AudioCapturers will be
// bindable to a set of inputs/outputs/renderers, so the concept of a
// "native" stream type will go away. Mechanisms will need to be put in
// place to allow users to enumerate the configuration of these bind-able
// endpoints (and perhaps to exercise control over them), but it will be
// the user of the AudioCapturer's job to specify the format they want.
GetStreamType() -> (StreamType stream_type);