shithub: opus

--- a/doc/draft-ietf-codec-oggopus.xml

+++ b/doc/draft-ietf-codec-oggopus.xml

@@ -101,7 +101,7 @@

  stream, to aid seeking.

 A single page can contain up to 65,025 octets of packet data from up to 255

  different packets.

-Packets may be split arbitrarily across pages, and continued from one page to

+Packets MAY be split arbitrarily across pages, and continued from one page to

  the next (allowing packets much larger than would fit on a single page).

 Each page contains 'lacing values' that indicate how the data is partitioned

  into packets, allowing a demuxer to recover the packet boundaries without

@@ -110,7 +110,7 @@

  lacing value corresponding to that packet.

 </t>

<t>

-This encapsulation defines the required contents of the packet data, including

+This encapsulation defines the contents of the packet data, including

  the necessary headers, the organization of those packets into a logical

  stream, and the interpretation of the codec-specific granule position field.

 It does not attempt to describe or specify the existing Ogg container format.

@@ -150,7 +150,7 @@

  (ID) header, which uniquely identifies a stream as Opus audio.

 The format of this header is defined in <xref target="id_header"/>.

 It MUST be placed alone (without any other packet data) on the first page of

- the logical Ogg bitstream, and must complete on that page.

+ the logical Ogg bitstream, and MUST complete on that page.

 This page MUST have its 'beginning of stream' flag set.

 </t>

<t>

@@ -166,7 +166,7 @@

 All subsequent pages are audio data pages, and the Ogg packets they contain are

  audio data packets.

 Each audio data packet contains one Opus packet for each of N different

- streams, where N is typically one for mono or stereo, but may be greater than

+ streams, where N is typically one for mono or stereo, but MAY be greater than

  one for multichannel audio.

 The value N is specified in the ID header (see

  <xref target="channel_mapping"/>), and is fixed over the entire length of the

@@ -201,10 +201,10 @@

 A decoder MUST treat a zero-octet audio data packet as if it were an Opus

  packet with an illegal TOC sequence.

 The last page SHOULD have the 'end of stream' flag set, but implementations

- should be prepared to deal with truncated streams that do not have a page

+ need to be prepared to deal with truncated streams that do not have a page

  marked 'end of stream'.

 The final packet on the last page SHOULD NOT be a continued packet, i.e., the

- final lacing value should be less than 255.

+ final lacing value SHOULD be less than 255.

 There MUST NOT be any more pages in an Opus logical bitstream after a page

  marked 'end of stream'.

 </t>

@@ -230,7 +230,7 @@

 </t>

<t>

-The duration of an Opus packet may be any multiple of 2.5&nbsp;ms, up to a

+The duration of an Opus packet can be any multiple of 2.5&nbsp;ms, up to a

  maximum of 120&nbsp;ms.

 This duration is encoded in the TOC sequence at the beginning of each packet.

 The number of samples returned by a decoder corresponds to this duration

@@ -330,7 +330,7 @@

  gap.

 This also requires four bytes to describe the synthesized packet data (two

  bytes for a CBR code 3 and one byte each for two code 0 packets) but three

- bytes of Ogg lacing overhead are required to mark the packet boundaries.

+ bytes of Ogg lacing overhead are necessary to mark the packet boundaries.

 At 0.6 kbps, this is still a minimal bitrate impact over a naive, low quality

  solution.

 </t>

@@ -349,7 +349,7 @@

 There is some amount of latency introduced during the decoding process, to

  allow for overlap in the CELT mode, stereo mixing in the SILK mode, and

  resampling.

-The encoder may introduce additional latency through its own resampling

+The encoder introduces additional latency through its own resampling

  and analysis (though the exact amount is not specified).

 Therefore, the first few samples produced by the decoder do not correspond to

  real input audio, but are instead composed of padding inserted by the encoder

@@ -366,7 +366,7 @@

  beginning of the stream.

 This amount MAY not be a multiple of 2.5&nbsp;ms, MAY be smaller than a single

  packet, or MAY span the contents of several packets.

-These samples are not valid audio, and should not be played.

+These samples are not valid audio, and SHOULD not be played.

 </t>

<t>

@@ -431,12 +431,12 @@

<t>

 Vorbis streams use a granule position smaller than the number of audio samples

  contained in the first audio data page to indicate that some of those samples

- must be trimmed from the output (see <xref target="vorbis-trim"/>).

+ are trimmed from the output (see <xref target="vorbis-trim"/>).

 However, to do so, Vorbis requires that the first audio data page contains

  exactly two packets, in order to allow the decoder to perform PCM position

  adjustments before needing to return any PCM data.

 Opus uses the pre-skip mechanism for this purpose instead, since the encoder

- may introduce more than a single packet's worth of latency, and since very

+ MAY introduce more than a single packet's worth of latency, and since very

  large packets in streams with a very large number of channels might not fit

  on a single page.

 </t>

@@ -470,11 +470,11 @@

  beginning of a stream to be cropped or a live stream to be joined without

  rewriting the granule position of all the remaining pages.

 This means that the PCM sample position just before the first sample to be

- played may be larger than '0'.

+ played MAY be larger than '0'.

 Synchronization when multiplexing with other logical streams still uses the PCM

  sample position relative to '0' to compute sample times.

 This does not affect the behavior of pre-skip: exactly 'pre-skip' samples

- should be skipped from the beginning of the decoded output, even if the

+ SHOULD be skipped from the beginning of the decoded output, even if the

  initial PCM sample position is greater than zero.

 </t>

@@ -482,7 +482,7 @@

 On the other hand, a granule position that is smaller than the number of

  decoded samples prevents a demuxer from working backwards to assign each

  packet or each individual sample a valid granule position, since granule

- positions must be non-negative.

+ positions are non-negative.

 A decoder MUST reject as invalid any stream where the granule position is

  smaller than the number of samples contained in packets that complete on the

  first audio data page with a completed packet, unless that page has the 'end

@@ -494,7 +494,7 @@

<t>

 If that page has the 'end of stream' flag set, a demuxer MUST reject as invalid

  any stream where its granule position is smaller than the 'pre-skip' amount.

-This would indicate that more samples should be skipped from the initial

+This would indicate that there are more samples to be skipped from the initial

  decoded output than exist in the stream.

 If the granule position is smaller than the number of decoded samples produced

  by the packets that complete on that page, then a demuxer MUST use an initial

@@ -528,8 +528,8 @@

 If the point 80&nbsp;ms prior to the seek target comes before the initial PCM

  sample position, the decoder SHOULD start decoding from the beginning of the

  stream, applying pre-skip as normal, regardless of whether the pre-skip is

- larger or smaller than 80&nbsp;ms, and then continue to discard the samples

- required to reach the seek target (if any).

+ larger or smaller than 80&nbsp;ms, and then continue to discard samples

+ to reach the seek target (if any).

 </t>

 </section>

@@ -634,7 +634,7 @@

 <vspace blankLines="1"/>

 Opus can switch between internal audio bandwidths of 4, 6, 8, 12, and

  20&nbsp;kHz.

-Each packet in the stream may have a different audio bandwidth.

+Each packet in the stream can have a different audio bandwidth.

 Regardless of the audio bandwidth, the reference decoder supports decoding any

  stream at a sample rate of 8, 12, 16, 24, or 48&nbsp;kHz.

 The original sample rate of the encoder input is not preserved by the lossy

@@ -653,7 +653,7 @@

 </list>

 However, the 'Input Sample Rate' field allows the encoder to pass the sample

  rate of the original input stream as metadata.

-This may be useful when the user requires the output sample rate to match the

+This is useful when the user requires the output sample rate to match the

  input sample rate.

 For example, a non-player decoder writing PCM format samples to disk might

  choose to resample the output audio back to the original input sample rate to

@@ -686,7 +686,7 @@

 </postamble>

 </figure>

 <vspace blankLines="1"/>

-Virtually all players and media frameworks should apply it by default.

+Virtually all players and media frameworks SHOULD apply it by default.

 If a player chooses to apply any volume adjustment or gain modification, such

  as the R128_TRACK_GAIN (see <xref target="comment_header"/>), the adjustment

  MUST be applied in addition to this output gain in order to achieve playback

@@ -694,9 +694,9 @@

 <vspace blankLines="1"/>

 An encoder SHOULD set this field to zero, and instead apply any gain prior to

  encoding, when this is possible and does not conflict with the user's wishes.

-The output gain should only be nonzero when the gain is adjusted after

- encoding, or when the user wishes to adjust the gain for playback while

- preserving the ability to recover the original signal amplitude.

+A nonzero output gain indicates the gain was adjusted after encoding, or that

+ a user wished to adjust the gain for playback while preserving the ability

+ to recover the original signal amplitude.

 <vspace blankLines="1"/>

 Although the output gain has enormous range (+/- 128 dB, enough to amplify

  inaudible sounds to the threshold of physical pain), most applications can

@@ -769,7 +769,7 @@

 <t><spanx style="strong">Stream Count</spanx> 'N' (8 bits, unsigned):

 <vspace blankLines="1"/>

 This is the total number of streams encoded in each Ogg packet.

-This value is required to correctly parse the packed Opus packets inside an

+This value is necessary to correctly parse the packed Opus packets inside an

  Ogg packet, as described in <xref target="packet_organization"/>.

 This value MUST NOT be zero, as without at least one Opus packet with a valid

  TOC sequence, a demuxer cannot recover the duration of an Ogg packet.

@@ -778,7 +778,7 @@

 <vspace blankLines="1"/>

 </t>

 <t><spanx style="strong">Coupled Stream Count</spanx> 'M' (8 bits, unsigned):

-This is the number of streams whose decoders should be configured to produce

+This is the number of streams whose decoders are to be configured to produce

  two channels.

 This MUST be no larger than the total number of streams, N.

 <vspace blankLines="1"/>

@@ -804,7 +804,7 @@

 </t>

 <t><spanx style="strong">Channel Mapping</spanx> (8*C bits):

 This contains one octet per output channel, indicating which decoded channel

- should be used for each one.

+ to be used for each one.

 Let 'index' be the value of this octet for a particular output channel.

 This value MUST either be smaller than (M+N), or be the special value 255.

 If 'index' is less than 2*M, the output MUST be taken from decoding stream

@@ -830,7 +830,7 @@

<t>

 After producing the output channels, the channel mapping family determines the

  semantic meaning of each one.

-Currently there are three defined mapping families, although more may be added.

+There are three defined mapping families in this specification.

 </t>

 <section anchor="channel_mapping_0" title="Channel Mapping Family 0">

@@ -1118,7 +1118,7 @@

 <vspace blankLines="1"/>

 This is a simple human-readable tag for vendor information, encoded as a UTF-8

  string&nbsp;<xref target="RFC3629"/>.

-No terminating null octet is required.

+No terminating null octet is necessary.

 <vspace blankLines="1"/>

 This tag is intended to identify the codec encoder and encapsulation

  implementations, for tracing differences in technical behavior.

@@ -1217,7 +1217,7 @@

  <spanx style="emph">in addition</spanx> to the 'output gain' value.

 If a tool modifies the ID header's 'output gain' field, it MUST also update or

  remove the R128_TRACK_GAIN and R128_ALBUM_GAIN comment tags if present.

-An encoder should assume that by default tools will respect the 'output gain'

+An encoder SHOULD assume that by default tools will respect the 'output gain'

  field, and not the comment tag.

 </t>

<t>

@@ -1241,8 +1241,8 @@

 Technically, valid Opus packets can be arbitrarily large due to the padding

  format, although the amount of non-padding data they can contain is bounded.

 These packets might be spread over a similarly enormous number of Ogg pages.

-Encoders SHOULD use no more padding than required to make a variable bitrate

- (VBR) stream constant bitrate (CBR).

+Encoders SHOULD use no more padding than is necessary to make a variable

+ bitrate (VBR) stream constant bitrate (CBR).

 Decoders SHOULD avoid attempting to allocate excessive amounts of memory when

  presented with a very large packet.

 The presence of an extremely large packet in the stream could indicate a

@@ -1285,7 +1285,7 @@

 <section anchor="encoder" title="Encoder Guidelines">

<t>

-When encoding Opus streams, Ogg encoders should take into account the

+When encoding Opus streams, Ogg encoders SHOULD take into account the

  algorithmic delay of the Opus encoder.

 </t>

 <figure align="center">

@@ -1379,8 +1379,8 @@

  opus_encoder_ctl(encoder_state, OPUS_SET_PREDICTION_DISABLED, 1);

 ]]></artwork>

 <postamble>

-Prediction should be enabled again before resuming normal encoding, even

- after a reset.

+For best results, this implementation requires that prediction be explicitly

+ enabled again before resuming normal encoding, even after a reset.

 </postamble>

 </figure>

@@ -1407,16 +1407,16 @@

 This is just as much a problem for the container as it is for the codec itself.

 It is extremely important for the decoder to be robust against malicious

  payloads.

-Malicious payloads must not cause the decoder to overrun its allocated memory

+Malicious payloads MUST NOT cause the decoder to overrun its allocated memory

  or to take an excessive amount of resources to decode.

 Although problems in encoders are typically rarer, the same applies to the

  encoder.

-Malicious audio streams must not cause the encoder to misbehave because this

+Malicious audio streams MUST NOT cause the encoder to misbehave because this

  would allow an attacker to attack transcoding gateways.

 </t>

<t>

-Like most other container formats, Ogg Opus streams should not be used with

+Like most other container formats, Ogg Opus streams SHOULD not be used with

  insecure ciphers or cipher modes that are vulnerable to known-plaintext

  attacks.

 Elements such as the Ogg page capture pattern and the magic signatures in the