Patent
1995-08-30
1998-04-14
MacDonald, Allen R.
395 214, 395 292, G10L 702, H04B 166
Patent
active
057403171
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method of determining the global masking threshold in a bit rate reducing source coding process.
2. Background Information
To code digital audio signals by means of bit rate reducing coding methods, WO 88/04,117 discloses the calculation of the spectral masking threshold in order to obtain a requantization rule.
Since the signals to be transmitted are not composed of only a single tone but of a plurality of harmonics, the masking thresholds created by such signals differ considerably. Their calculation requires a consideration of all relevant tonal maskers and of all relevant noise maskers, each having frequency and level specific masking edges. Such an extensive consideration requires a correspondingly high calculating effort in the source coder which is justified only for a computer simulation but not for a real time realization.
SUMMARY OF THE INVENTION
In contrast thereto, it is the object of the invention to reduce the calculating effort for a bit rate reducing source coding process particularly for real time applications.
Advantageous features and modifications of the method according to the invention are defined in the following description.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described in greater detail with reference to the drawings, in which:
FIG. 1 depicts a block circuit diagram of a source coder for implementing the method according to the invention;
FIG. 2 depicts a frequency diagram including three maskers and the resting threshold whose joint masking effect results in the global masking threshold determined according to the invention.
FIGS. 3-7 are flow charts of the inventive method.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
In the block circuit diagram of FIG. 1, the digitized audio signal 1 at the input is fed, in the case of sub-band coding, to a polyphase filter bank 10 which produces sub-band sampling values 2 (step 1180). In the case of transformation coding, filter bank 10 is replaced by a time/frequency transformation stage which produces discrete, spectral sampling values, for example, corresponding to a cosine or a fast Fourier transformation. Sampling values 2 are requantized in a quantizing stage 20 according to their permissible quantizing noise as determined by a coding and requantizing control signal 7 (step 1190). In order to form an output signal 8, control signal 7 is fed, together with the requantized sampling values 3, to a multiplexer 70 which inserts signals 3 and 7 into a time multiplex frame depending on the bit rate reduction method employed (step 1200).
The digitized audio signal 1 at the input is also fed to a transformation stage 40 which, in the case of sub-band coding, produces discrete spectral sampling values 5 (step 1280). In the case of transformation coding, the spectral sampling values determined in the time/frequency transformation stage can be employed as sampling values 5 (path 2a shown in dashed lines). According to a procedure (step 1220) specific to the invention to be described in greater detail below, a stage 50 calculates the global masking threshold 6 from sampling values 5 and possibly the maximum signal levels 4.
For sub-band coding, a stage 30 additionally determines the maximum signal levels 4 in the individual sub-bands from the sampling values 2.
In a stage 60, the above-mentioned coding and requantizing control signal 7 is produced from the global masking threshold 6. Stage 60 is described in FIG. 3, information blocks 5.5 and 5.3, of the above-mentioned WO 88/04,117 which is expressly referred to. In the mentioned information block 5.5, the relationship between maximum occurring (masking) sub-band level and minimum global masking threshold is determined (according to permissible quantizing noise), from which, in the subsequent information block 5.3 , the sub-band association of the quantization (=resolution) is calculated.
The calculation of global masking threshold 6 (step 1220) will now be described in greater detai
REFERENCES:
patent: 4862346 (1989-08-01), Wagner et al.
patent: 4972484 (1990-11-01), Theile et al.
patent: 5353375 (1994-10-01), Gotu et al.
Brefort Andreas
Groh Jens
Krafft Wolfgang
Link Martin
Rosinski Klaus
Institut Fuer Rundfunktechnik GmbH
MacDonald Allen R.
Storm Donald L.
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