Subject: [codec] [Errata Held for Document Update] RFC6716 (4392)

From: RFC Errata System <[email-masked]>
Date: Mon, 15 Jun 2015 20:18:17 -0700 (PDT)

The following errata report has been held for document update 
for RFC6716, "Definition of the Opus Audio Codec". 

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You may review the report below and at:
http://www.rfc-editor.org/errata_search.php?rfc=6716&eid=4392

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Status: Held for Document Update
Type: Technical

Reported by: Peter Budny <[email-masked]>
Date Reported: 2015-06-15
Held by: Ben Campbell (IESG)

Section: 2

Original Text
-------------
The Opus codec scales from 6 kbit/s narrowband mono speech to
510 kbit/s fullband stereo music, with algorithmic delays ranging
from 5 ms to 65.2 ms.

(further in the same section)

To
compensate for the different look-ahead required by each layer, the
CELT encoder input is delayed by an additional 2.7 ms.

Corrected Text
--------------
The Opus codec scales from 6 kbit/s narrowband mono speech to
510 kbit/s fullband stereo music, with algorithmic delays ranging
from 5 ms to 66.5 ms.

(further in the same section)

To
compensate for the different look-ahead required by each layer, the
CELT encoder input is delayed by an additional 4 ms.

Notes
-----
For the latter text, the delays for the CELT and SILK layers must match.  SILK has a "5 ms look-ahead for noise shaping estimation", and "1.5 ms delay for sampling rate conversion", totaling 6.5 ms.  CELT has a "2.5 ms look-ahead due to the overlapping MDCT windows".  Thus, the amount of delay needed to align CELT with SILK is 6.5ms - 2.5ms = 4ms.

The text at the beginning of the section must reflect this as well.  The "algorithmic delays" reported apparently include framing (minimum frame size 2.5ms + look-ahead delay in CELT-only mode 2.5ms = 5ms).  In that case, the maximum delay is the maximum frame size (60ms) + the maximum delay for look-ahead and resampling (6.5ms) = 66.5ms.

Confirmed by author ("Jmvalin") at https://en.wikipedia.org/wiki/Talk:Opus_(audio_format)/Archive_1#Codec_delay_values

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RFC6716 (draft-ietf-codec-opus-16)
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Title               : Definition of the Opus Audio Codec
Publication Date    : September 2012
Author(s)           : JM. Valin, K. Vos, T. Terriberry
Category            : PROPOSED STANDARD
Source              : Internet Wideband Audio Codec RAI
Area                : Real-time Applications and Infrastructure
Stream              : IETF
Verifying Party     : IESG