Dolby Digital (AC-3)
Full Details of Dolby Digital (AC-3) System
Man Listening with Headphones
Man Listening Dobly Digital with Headphones
Dolby Digital, also known as AC-3, is a digital audio compression technology developed by Dolby Laboratories. It allows the reduction of the amount of data needed to produce high-quality sound. Dolby Digital is widely used in cinema, television, DVDs, Blu-rays, and other media.
Full Details of Dolby Digital (AC-3) System
Overview: Dolby Digital compresses audio using a lossy algorithm, reducing the data rate without significantly affecting sound quality. It supports up to six discrete channels of sound: five full-bandwidth channels (left, center, right, left surround, right surround) and one low-frequency effects (LFE) channel, often referred to as 5.1 channel sound.
Technical Details:
Bit Rate: Dolby Digital supports bit rates from 32 kbps to 640 kbps. Common bit rates are 384 kbps for DVDs and 448 kbps for Blu-rays.
Sample Rate: It supports sample rates of 32 kHz, 44.1 kHz, and 48 kHz.
Channels: Up to six discrete audio channels (5.1).
Compression Process:
Downmixing: If the source has more than six channels, it can be downmixed.
Partitioning: Audio is partitioned into frames, each containing a fixed number of samples.
Transform: Each partition is transformed from the time domain to the frequency domain using Modified Discrete Cosine Transform (MDCT).
Bit Allocation: Bits are allocated to different frequency bands based on psychoacoustic models.
Quantization: The frequency coefficients are quantized.
Encoding: Quantized coefficients are encoded using Huffman coding or other methods.
Formulas Used in Dolby Digital (AC-3)
1. Modified Discrete Cosine Transform = MDCT
3. Psychoacoustic Model: The psychoacoustic model is complex and involves determining the masking threshold:
4. Bit Allocation: The bit allocation is based on the masking threshold and the available bit rate. The specific formula can vary but typically involves:
Key Features
MDCT Calculation: Takes time-domain signal and transform length as input and computes the MDCT coefficients.
IMDCT Calculation: Takes MDCT coefficients and transform length as input and reconstructs the time-domain signal.
Bit Allocation Calculation: Takes total available bit rate and signal-to-mask ratios as input and computes the bit allocation.
Here’s a simple UI to visualize the Dolby Digital (AC-3) process: