Sound signal downmixing method, sound signal coding method, sound signal downmixing apparatus, sound signal coding apparatus, program and recording medium

A sound signal downmix apparatus configured to obtain a downmix signal that is a monaural sound signal from input sound signals of N channels, N being an integer of three or greater, the sound signal downmix apparatus comprising:

• an inter-channel relationship information obtaining unit configured to obtain an inter-channel correlation value and preceding channel information of every pair of two channels included in the N channels, the inter-channel correlation value being a value indicating a degree of a correlation between input sound signals of the two channels, the preceding channel information being information indicating which of the input sound signals of the two channels is preceding;
• and
• a downmix unit configured to obtain the downmix signal by weighting and adding the input sound signals of the N channels, the input sound signal of each channel being weighted based on the inter-channel correlation value and the preceding channel information such that the larger a correlation with an input sound signal of a preceding channel that precedes the channel, the smaller a weight, whereas the larger a correlation with an input sound signal of a succeeding channel that succeeds the channel, the larger the weight, wherein
• the inter-channel relationship information obtaining unit includes
• a channel sorting unit configured to sequentially perform sorting in an order from a first channel such that an adjacent channel is a channel with a most similar input sound signal among remaining channels, and obtain a first sorted input sound signal to an Nth sorted input sound signal that are signals after the sorting of the N channels, and first original channel information to Nth original channel information of the N channels for the sorted input sound signals, the first original channel information to the Nth original channel information being channel numbers of the N channels for the input sound signals,
• an inter-adjacent-channel relationship information estimation unit configured to obtain an inter-channel correlation value and an inter-channel time difference of every pair of two channels after the sorting with adjacent channel numbers after the sorting among the first to Nth sorted input sound signals, and
• an inter-channel relationship information complement unit configured to
• obtain an inter-channel correlation value of every pair of two channels after the sorting with non-adjacent channel numbers after the sorting from the inter-channel correlation value of every pair of two channels after the sorting with adjacent channel numbers after the sorting,
• obtain the inter-channel correlation value between the input sound signals of every pair of two channels included in the N channels by associating the inter-channel correlation value of every pair of channels after the sorting with a pair of channels for the input sound signals of the N channels by using the original channel information,
• obtain an inter-channel time difference of every pair of two channels after the sorting with non-adjacent channel numbers after the sorting from the inter-channel time difference of every pair of two channels after the sorting with adjacent channel numbers after the sorting, and
• obtain preceding channel information of every pair of two channels included in the N channels by establishing an association with a pair of channels for the input sound signals of the N channels by using the original channel information from the inter-channel time difference of every pair of channels after the sorting, and obtaining the preceding channel information based on whether the inter-channel time difference is positive, negative or zero,
• two channel numbers of every pair of two channels after the sorting with adjacent channel numbers after the sorting are denoted as i and i+1, i being an integer from 1 to N−1,
• the inter-channel correlation value of every pair of two channels after the sorting with adjacent channel numbers after the sorting is denoted as γ′i(i+1),
• the inter-channel time difference of every pair of two channels after the sorting with adjacent channel numbers after the sorting is denoted as τ′i(i+1),
• two channel numbers of every pair of two channels after the sorting with non-adjacent channel numbers after the sorting are denoted as n and m, n being an integer from 1 to N−2, m being an integer from n+2 to N,
• the inter-channel correlation value of every pair of two channels after the sorting with non-adjacent channel numbers after the sorting is denoted as γ′nm, and
• the inter-channel time difference of every pair of two channels after the sorting with non-adjacent channel numbers after the sorting is denoted as τ′nm,
• the inter-channel correlation value γ′nm of every pair of two channels after the sorting with non-adjacent channel numbers after the sorting is a value that has a monotonically non-decreasing relationship with every of one or more of the inter-channel correlation values γ′i(i+1) including a minimum value of the inter-channel correlation values γ′i(i+1) of the pairs of two channels with adjacent channel numbers after the sorting, i of the inter-channel correlation values γ′i(i+1) being from n to m−1, and
• the inter-channel time difference τ′nm of every pair of two channels after the sorting with non-adjacent channel numbers after the sorting is a value obtained by adding up all of the inter-channel time differences τ′i(i+1) of the pairs of two channels with adjacent channel numbers after the sorting, i of the inter-channel time differences τ′i(i+1) being from n to m−1.