$Id: 06-floor0.xml 10424 2005-11-23 08:44:18Z xiphmont $

Vorbis floor type zero uses Line Spectral Pair (LSP, also alternately known as Line Spectral Frequency or LSF) representation to encode a smooth spectral envelope curve as the frequency response of the LSP filter. This representation is equivalent to a traditional all-pole infinite impulse response filter as would be used in linear predictive coding; LSP representation may be converted to LPC representation and vice-versa.

Floor zero configuration consists of six integer fields and a list of VQ codebooks for use in coding/decoding the LSP filter coefficient values used by each frame.

Configuration information for instances of floor zero decodes from the codec setup header (third packet). configuration decode proceeds as follows: 1) [floor0_order] = read an unsigned integer of 8 bits 2) [floor0_rate] = read an unsigned integer of 16 bits 3) [floor0_bark_map_size] = read an unsigned integer of 16 bits 4) [floor0_amplitude_bits] = read an unsigned integer of six bits 5) [floor0_amplitude_offset] = read an unsigned integer of eight bits 6) [floor0_number_of_books] = read an unsigned integer of four bits and add 1 7) if any of [floor0_order], [floor0_rate], [floor0_bark_map_size], [floor0_amplitude_bits], [floor0_amplitude_offset] or [floor0_number_of_books] are less than zero, the stream is not decodable 8) array [floor0_book_list] = read a list of [floor0_number_of_books] unsigned integers of eight bits each; An end-of-packet condition during any of these bitstream reads renders this stream undecodable. In addition, any element of the array [floor0_book_list] that is greater than the maximum codebook number for this bitstream is an error condition that also renders the stream undecodable.

Extracting a floor0 curve from an audio packet consists of first decoding the curve amplitude and [floor0_order] LSP coefficient values from the bitstream, and then computing the floor curve, which is defined as the frequency response of the decoded LSP filter. Packet decode proceeds as follows: 1) [amplitude] = read an unsigned integer of [floor0_amplitude_bits] bits 2) if ( [amplitude] is greater than zero ) { 3) [coefficients] is an empty, zero length vector 4) [booknumber] = read an unsigned integer of ilog( [floor0_number_of_books] ) bits 5) if ( [booknumber] is greater than the highest number decode codebook ) then packet is undecodable 6) [last] = zero; 7) vector [temp_vector] = read vector from bitstream using codebook number [floor0_book_list] element [booknumber] in VQ context. 8) add the scalar value [last] to each scalar in vector [temp_vector] 9) [last] = the value of the last scalar in vector [temp_vector] 10) concatenate [temp_vector] onto the end of the [coefficients] vector 11) if (length of vector [coefficients] is less than [floor0_order], continue at step 6 } 12) done. Take note of the following properties of decode: An [amplitude] value of zero must result in a return code that indicates this channel is unused in this frame (the output of the channel will be all-zeroes in synthesis). Several later stages of decode don't occur for an unused channel. An end-of-packet condition during decode should be considered a nominal occruence; if end-of-packet is reached during any read operation above, floor decode is to return 'unused' status as if the [amplitude] value had read zero at the beginning of decode. The book number used for decode can, in fact, be stored in the bitstream in ilog( [floor0_number_of_books] - 1 ) bits. Nevertheless, the above specification is correct and values greater than the maximum possible book value are reserved. The number of scalars read into the vector [coefficients] may be greater than [floor0_order], the number actually required for curve computation. For example, if the VQ codebook used for the floor currently being decoded has a [codebook_dimensions] value of three and [floor0_order] is ten, the only way to fill all the needed scalars in [coefficients] is to to read a total of twelve scalars as four vectors of three scalars each. This is not an error condition, and care must be taken not to allow a buffer overflow in decode. The extra values are not used and may be ignored or discarded.

Given an [amplitude] integer and [coefficients] vector from packet decode as well as the [floor0_order], [floor0_rate], [floor0_bark_map_size], [floor0_amplitude_bits] and [floor0_amplitude_offset] values from floor setup, and an output vector size [n] specified by the decode process, we compute a floor output vector. If the value [amplitude] is zero, the return value is a length [n] vector with all-zero scalars. Otherwise, begin by assuming the following definitions for the given vector to be synthesized: [lsp map equation] The above is used to synthesize the LSP curve on a Bark-scale frequency axis, then map the result to a linear-scale frequency axis. Similarly, the below calculation synthesizes the output LSP curve [output] on a log (dB) amplitude scale, mapping it to linear amplitude in the last step: [i] = 0 if ( [floor0_order] is odd ) { calculate [p] and [q] according to: [equation for odd lsp] } else [floor0_order] is even { calculate [p] and [q] according to: [equation for even lsp] } calculate [linear_floor_value] according to: [expression for floorval] [iteration_condition] = map element [i] [output] element [i] = [linear_floor_value] increment [i] if ( map element [i] is equal to [iteration_condition] ) continue at step 5 if ( [i] is less than [n] ) continue at step 2 done