Ogg Vorbis I format specification: floor type 0 setup and decode

Overview

Floor 0 format

header decode

  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;

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) [lastval] = zero;
       7) vector [temp_vector] = read vector from bitstream using codebook number [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.
 

• 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.

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:

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:

  1) [i] = 0
  2) if ( [floor0_order] is odd ) {

       3) calculate [p] and [q] according to: 

          

     } else [floor0_order] is even {

       4) calculate [p] and [q] according to: 

          

     }

  5) calculate [linear_floor_value] according to:

     

  6) [iteration_condition] = map element [i]
  7) [output] element [i] = [linear_floor_value]
  8) increment [i]
  9) if ( map element [i] is equal to [iteration_condition] ) continue at step 7
 10) if ( [i] is less than [n] ) continue at step 2
 11) done