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$Id: 04-codec.xml 10466 2005-11-28 00:34:44Z giles $

This document serves as the top-level reference document for the bit-by-bit decode specification of Vorbis I. This document assumes a high-level understanding of the Vorbis decode process, which is provided in . covers reading and writing bit fields from and to bitstream packets.

A Vorbis bitstream begins with three header packets. The header packets are, in order, the identification header, the comments header, and the setup header. All are required for decode compliance. An end-of-packet condition during decoding the first or third header packet renders the stream undecodable. End-of-packet decoding the comment header is a non-fatal error condition.

Each header packet begins with the same header fields. 1) [packet_type] : 8 bit value 2) 0x76, 0x6f, 0x72, 0x62, 0x69, 0x73: the characters 'v','o','r','b','i','s' as six octets Decode continues according to packet type; the identification header is type 1, the comment header type 3 and the setup header type 5 (these types are all odd as a packet with a leading single bit of '0' is an audio packet). The packets must occur in the order of identification, comment, setup.

The identification header is a short header of only a few fields used to declare the stream definitively as Vorbis, and provide a few externally relevant pieces of information about the audio stream. The identification header is coded as follows: 1) [vorbis_version] = read 32 bits as unsigned integer 2) [audio_channels] = read 8 bit integer as unsigned 3) [audio_sample_rate] = read 32 bits as unsigned integer 4) [bitrate_maximum] = read 32 bits as signed integer 5) [bitrate_nominal] = read 32 bits as signed integer 6) [bitrate_minimum] = read 32 bits as signed integer 7) [blocksize_0] = 2 exponent (read 4 bits as unsigned integer) 8) [blocksize_1] = 2 exponent (read 4 bits as unsigned integer) 9) [framing_flag] = read one bit [vorbis_version] is to read '0' in order to be compatible with this document. Both [audio_channels] and [audio_sample_rate] must read greater than zero. Allowed final blocksize values are 64, 128, 256, 512, 1024, 2048, 4096 and 8192 in Vorbis I. [blocksize_0] must be less than or equal to [blocksize_1]. The framing bit must be nonzero. Failure to meet any of these conditions renders a stream undecodable. The bitrate fields above are used only as hints. The nominal bitrate field especially may be considerably off in purely VBR streams. The fields are meaningful only when greater than zero. All three fields set to the same value implies a fixed rate, or tightly bounded, nearly fixed-rate bitstream Only nominal set implies a VBR or ABR stream that averages the nominal bitrate Maximum and or minimum set implies a VBR bitstream that obeys the bitrate limits None set indicates the encoder does not care to speculate.

Comment header decode and data specification is covered in .

Vorbis codec setup is configurable to an extreme degree: [decoder pipeline configuration] The setup header contains the bulk of the codec setup information needed for decode. The setup header contains, in order, the lists of codebook configurations, time-domain transform configurations (placeholders in Vorbis I), floor configurations, residue configurations, channel mapping configurations and mode configurations. It finishes with a framing bit of '1'. Header decode proceeds in the following order:

[vorbis_codebook_count] = read eight bits as unsigned integer and add one Decode [vorbis_codebook_count] codebooks in order as defined in . Save each configuration, in order, in an array of codebook configurations [vorbis_codebook_configurations].

These hooks are placeholders in Vorbis I. Nevertheless, the configuration placeholder values must be read to maintain bitstream sync. [vorbis_time_count] = read 6 bits as unsigned integer and add one read [vorbis_time_count] 16 bit values; each value should be zero. If any value is nonzero, this is an error condition and the stream is undecodable.

Vorbis uses two floor types; header decode is handed to the decode abstraction of the appropriate type. [vorbis_floor_count] = read 6 bits as unsigned integer and add one For each [i] of [vorbis_floor_count] floor numbers: read the floor type: vector [vorbis_floor_types] element [i] = read 16 bits as unsigned integer If the floor type is zero, decode the floor configuration as defined in ; save this configuration in slot [i] of the floor configuration array [vorbis_floor_configurations]. If the floor type is one, decode the floor configuration as defined in ; save this configuration in slot [i] of the floor configuration array [vorbis_floor_configurations]. If the the floor type is greater than one, this stream is undecodable; ERROR CONDITION

Vorbis uses three residue types; header decode of each type is identical. [vorbis_residue_count] = read 6 bits as unsigned integer and add one For each of [vorbis_residue_count] residue numbers: read the residue type; vector [vorbis_residue_types] element [i] = read 16 bits as unsigned integer If the residue type is zero, one or two, decode the residue configuration as defined in ; save this configuration in slot [i] of the residue configuration array [vorbis_residue_configurations]. If the the residue type is greater than two, this stream is undecodable; ERROR CONDITION

Mappings are used to set up specific pipelines for encoding multichannel audio with varying channel mapping applications. Vorbis I uses a single mapping type (0), with implicit PCM channel mappings. [vorbis_mapping_count] = read 6 bits as unsigned integer and add one For each [i] of [vorbis_mapping_count] mapping numbers: read the mapping type: 16 bits as unsigned integer. There's no reason to save the mapping type in Vorbis I. If the mapping type is nonzero, the stream is undecodable If the mapping type is zero: read 1 bit as a boolean flag if set, [vorbis_mapping_submaps] = read 4 bits as unsigned integer and add one if unset, [vorbis_mapping_submaps] = 1 read 1 bit as a boolean flag if set, square polar channel mapping is in use: [vorbis_mapping_coupling_steps] = read 8 bits as unsigned integer and add one for [j] each of [vorbis_mapping_coupling_steps] steps: vector [vorbis_mapping_magnitude] element [j]= read ilog([audio_channels] - 1) bits as unsigned integer vector [vorbis_mapping_angle] element [j]= read ilog([audio_channels] - 1) bits as unsigned integer the numbers read in the above two steps are channel numbers representing the channel to treat as magnitude and the channel to treat as angle, respectively. If for any coupling step the angle channel number equals the magnitude channel number, the magnitude channel number is greater than [audio_channels]-1, or the angle channel is greater than [audio_channels]-1, the stream is undecodable. if unset, [vorbis_mapping_coupling_steps] = 0 read 2 bits (reserved field); if the value is nonzero, the stream is undecodable if [vorbis_mapping_submaps] is greater than one, we read channel multiplex settings. For each [j] of [audio_channels] channels: vector [vorbis_mapping_mux] element [j] = read 4 bits as unsigned integer if the value is greater than the highest numbered submap ([vorbis_mapping_submaps] - 1), this in an error condition rendering the stream undecodable for each submap [j] of [vorbis_mapping_submaps] submaps, read the floor and residue numbers for use in decoding that submap: read and discard 8 bits (the unused time configuration placeholder) read 8 bits as unsigned integer for the floor number; save in vector [vorbis_mapping_submap_floor] element [j] verify the floor number is not greater than the highest number floor configured for the bitstream. If it is, the bitstream is undecodable read 8 bits as unsigned integer for the residue number; save in vector [vorbis_mapping_submap_residue] element [j] verify the residue number is not greater than the highest number residue configured for the bitstream. If it is, the bitstream is undecodable save this mapping configuration in slot [i] of the mapping configuration array [vorbis_mapping_configurations].

[vorbis_mode_count] = read 6 bits as unsigned integer and add one For each of [vorbis_mode_count] mode numbers: [vorbis_mode_blockflag] = read 1 bit [vorbis_mode_windowtype] = read 16 bits as unsigned integer [vorbis_mode_transformtype] = read 16 bits as unsigned integer [vorbis_mode_mapping] = read 8 bits as unsigned integer verify ranges; zero is the only legal value in Vorbis I for [vorbis_mode_windowtype] and [vorbis_mode_transformtype]. [vorbis_mode_mapping] must not be greater than the highest number mapping in use. Any illegal values render the stream undecodable. save this mode configuration in slot [i] of the mode configuration array [vorbis_mode_configurations]. read 1 bit as a framing flag. If unset, a framing error occurred and the stream is not decodable. After reading mode descriptions, setup header decode is complete.

Following the three header packets, all packets in a Vorbis I stream are audio. The first step of audio packet decode is to read and verify the packet type. A non-audio packet when audio is expected indicates stream corruption or a non-compliant stream. The decoder must ignore the packet and not attempt decoding it to audio.

read 1 bit [packet_type]; check that packet type is 0 (audio) read ilog([vorbis_mode_count]-1) bits [mode_number] decode blocksize [n] is equal to [blocksize_0] if [vorbis_mode_blockflag] is 0, else [n] is equal to [blocksize_1]. perform window selection and setup; this window is used later by the inverse MDCT: if this is a long window (the [vorbis_mode_blockflag] flag of this mode is set): read 1 bit for [previous_window_flag] read 1 bit for [next_window_flag] if [previous_window_flag] is not set, the left half of the window will be a hybrid window for lapping with a short block. See for an illustration of overlapping dissimilar windows. Else, the left half window will have normal long shape. if [next_window_flag] is not set, the right half of the window will be a hybrid window for lapping with a short block. See for an illustration of overlapping dissimilar windows. Else, the left right window will have normal long shape. if this is a short window, the window is always the same short-window shape. Vorbis windows all use the slope function y=sin(0.5 * π * sin^2((x+.5)/n * π)), where n is window size and x ranges 0...n-1, but dissimilar lapping requirements can affect overall shape. Window generation proceeds as follows: [window_center] = [n] / 2 if ([vorbis_mode_blockflag] is set and [previous_window_flag] is not set) then [left_window_start] = [n]/4 - [blocksize_0]/4 [left_window_end] = [n]/4 + [blocksize_0]/4 [left_n] = [blocksize_0]/2 else [left_window_start] = 0 [left_window_end] = [window_center] [left_n] = [n]/2 if ([vorbis_mode_blockflag] is set and [next_window_flag] is not set) then [right_window_start] = [n]*3/4 - [blocksize_0]/4 [right_window_end] = [n]*3/4 + [blocksize_0]/4 [right_n] = [blocksize_0]/2 else [right_window_start] = [window_center] [right_window_end] = [n] [right_n] = [n]/2 window from range 0 ... [left_window_start]-1 inclusive is zero for [i] in range [left_window_start] ... [left_window_end]-1, window([i]) = sin(.5 * π * sin^2( ([i]-[left_window_start]+.5) / [left_n] * .5 * π) ) window from range [left_window_end] ... [right_window_start]-1 inclusive is one for [i] in range [right_window_start] ... [right_window_end]-1, window([i]) = sin(.5 * π * sin^2( ([i]-[right_window_start]+.5) / [right_n] * .5 * π + .5 * π) ) window from range [right_window_start] ... [n]-1 is zero An end-of-packet condition up to this point should be considered an error that discards this packet from the stream. An end of packet condition past this point is to be considered a possible nominal occurrence.

From this point on, we assume out decode context is using mode number [mode_number] from configuration array [vorbis_mode_configurations] and the map number [vorbis_mode_mapping] (specified by the current mode) taken from the mapping configuration array [vorbis_mapping_configurations]. Floor curves are decoded one-by-one in channel order. For each floor [i] of [audio_channels] [submap_number] = element [i] of vector [vorbis_mapping_mux] [floor_number] = element [submap_number] of vector [vorbis_submap_floor] if the floor type of this floor (vector [vorbis_floor_types] element [floor_number]) is zero then decode the floor for channel [i] according to the if the type of this floor is one then decode the floor for channel [i] according to the save the needed decoded floor information for channel for later synthesis if the decoded floor returned 'unused', set vector [no_residue] element [i] to true, else set vector [no_residue] element [i] to false An end-of-packet condition during floor decode shall result in packet decode zeroing all channel output vectors and skipping to the add/overlap output stage.

A possible result of floor decode is that a specific vector is marked 'unused' which indicates that that final output vector is all-zero values (and the floor is zero). The residue for that vector is not coded in the stream, save for one complication. If some vectors are used and some are not, channel coupling could result in mixing a zeroed and nonzeroed vector to produce two nonzeroed vectors. for each [i] from 0 ... [vorbis_mapping_coupling_steps]-1 if either [no_residue] entry for channel ([vorbis_mapping_magnitude] element [i]) or channel ([vorbis_mapping_angle] element [i]) are set to false, then both must be set to false. Note that an 'unused' floor has no decoded floor information; it is important that this is remembered at floor curve synthesis time.

Unlike floors, which are decoded in channel order, the residue vectors are decoded in submap order. for each submap [i] in order from 0 ... [vorbis_mapping_submaps]-1 [ch] = 0 for each channel [j] in order from 0 ... [audio_channels] - 1 if channel [j] in submap [i] (vector [vorbis_mapping_mux] element [j] is equal to [i]) if vector [no_residue] element [j] is true vector [do_not_decode_flag] element [ch] is set else vector [do_not_decode_flag] element [ch] is unset increment [ch] [residue_number] = vector [vorbis_mapping_submap_residue] element [i] [residue_type] = vector [vorbis_residue_types] element [residue_number] decode [ch] vectors using residue [residue_number], according to type [residue_type], also passing vector [do_not_decode_flag] to indicate which vectors in the bundle should not be decoded. Correct per-vector decode length is [n]/2. [ch] = 0 for each channel [j] in order from 0 ... [audio_channels] if channel [j] is in submap [i] (vector [vorbis_mapping_mux] element [j] is equal to [i]) residue vector for channel [j] is set to decoded residue vector [ch] increment [ch]

for each [i] from [vorbis_mapping_coupling_steps]-1 descending to 0 [magnitude_vector] = the residue vector for channel (vector [vorbis_mapping_magnitude] element [i]) [angle_vector] = the residue vector for channel (vector [vorbis_mapping_angle] element [i]) for each scalar value [M] in vector [magnitude_vector] and the corresponding scalar value [A] in vector [angle_vector]: if ([M] is greater than zero) if ([A] is greater than zero) [new_M] = [M] [new_A] = [M]-[A] else [new_A] = [M] [new_M] = [M]+[A] else if ([A] is greater than zero) [new_M] = [M] [new_A] = [M]+[A] else [new_A] = [M] [new_M] = [M]-[A] set scalar value [M] in vector [magnitude_vector] to [new_M] set scalar value [A] in vector [angle_vector] to [new_A]

For each channel, synthesize the floor curve from the decoded floor information, according to packet type. Note that the vector synthesis length for floor computation is [n]/2. For each channel, multiply each element of the floor curve by each element of that channel's residue vector. The result is the dot product of the floor and residue vectors for each channel; the produced vectors are the length [n]/2 audio spectrum for each channel. One point is worth mentioning about this dot product; a common mistake in a fixed point implementation might be to assume that a 32 bit fixed-point representation for floor and residue and direct multiplication of the vectors is sufficient for acceptable spectral depth in all cases because it happens to mostly work with the current Xiph.Org reference encoder. However, floor vector values can span ~140dB (~24 bits unsigned), and the audio spectrum vector should represent a minimum of 120dB (~21 bits with sign), even when output is to a 16 bit PCM device. For the residue vector to represent full scale if the floor is nailed to -140dB, it must be able to span 0 to +140dB. For the residue vector to reach full scale if the floor is nailed at 0dB, it must be able to represent -140dB to +0dB. Thus, in order to handle full range dynamics, a residue vector may span -140dB to +140dB entirely within spec. A 280dB range is approximately 48 bits with sign; thus the residue vector must be able to represent a 48 bit range and the dot product must be able to handle an effective 48 bit times 24 bit multiplication. This range may be achieved using large (64 bit or larger) integers, or implementing a movable binary point representation.

Convert the audio spectrum vector of each channel back into time domain PCM audio via an inverse Modified Discrete Cosine Transform (MDCT). A detailed description of the MDCT is available in the paper The use of multirate filter banks for coding of high quality digital audio, by T. Sporer, K. Brandenburg and B. Edler. The window function used for the MDCT is the function described earlier.

Windowed MDCT output is overlapped and added with the right hand data of the previous window such that the 3/4 point of the previous window is aligned with the 1/4 point of the current window (as illustrated in ). The overlapped portion produced from overlapping the previous and current frame data is finished data to be returned by the decoder. This data spans from the center of the previous window to the center of the current window. In the case of same-sized windows, the amount of data to return is one-half block consisting of and only of the overlapped portions. When overlapping a short and long window, much of the returned range does not actually overlap. This does not damage transform orthogonality. Pay attention however to returning the correct data range; the amount of data to be returned is: window_blocksize(previous_window)/4+window_blocksize(current_window)/4 from the center (element windowsize/2) of the previous window to the center (element windowsize/2-1, inclusive) of the current window. Data is not returned from the first frame; it must be used to 'prime' the decode engine. The encoder accounts for this priming when calculating PCM offsets; after the first frame, the proper PCM output offset is '0' (as no data has been returned yet).

Vorbis I specifies only a channel mapping type 0. In mapping type 0, channel mapping is implicitly defined as follows for standard audio applications: one channel the stream is monophonic two channels the stream is stereo. channel order: left, right three channels the stream is a 1d-surround encoding. channel order: left, center, right four channels the stream is quadraphonic surround. channel order: front left, front right, rear left, rear right five channels the stream is five-channel surround. channel order: front left, front center, front right, rear left, rear right six channels the stream is 5.1 surround. channel order: front left, front center, front right, rear left, rear right, LFE greater than six channels channel use and order is defined by the application Applications using Vorbis for dedicated purposes may define channel mapping as seen fit. Future channel mappings (such as three and four channel Ambisonics) will make use of channel mappings other than mapping 0.