EVRC (Enhanced Variable Rate Codec)

The Enhanced Variable Rate Codec (EVRC) is a speech coding algorithm that was developed by Qualcomm Inc. in 1995. The codec was designed for use in CDMA (Code Division Multiple Access) digital cellular systems and is widely used in the United States for wireless communications.

Speech coding algorithms are used to compress audio signals for transmission over limited bandwidth networks. The EVRC codec uses a combination of linear predictive coding (LPC) and codebook quantization to achieve high-quality speech at low bit rates.

The EVRC codec operates at a variable bit rate, which means that the amount of data used to represent the speech signal can vary depending on the complexity of the signal. The codec uses a combination of three different modes to adjust the bit rate based on the characteristics of the input signal.

The first mode is called the Full Rate (FR) mode, which operates at a fixed bit rate of 8.55 kbps. This mode is used for high-quality speech signals and provides a sampling rate of 8 kHz with 13-bit PCM (Pulse Code Modulation) quantization.

The second mode is called the Half Rate (HR) mode, which operates at a fixed bit rate of 4.75 kbps. This mode is used for low-quality speech signals and provides a sampling rate of 8 kHz with 10-bit PCM quantization.

The third mode is called the Quarter Rate (QR) mode, which operates at a fixed bit rate of 2.4 kbps. This mode is used for very low-quality speech signals and provides a sampling rate of 4 kHz with 8-bit PCM quantization.

The EVRC codec uses a combination of linear predictive coding (LPC) and codebook quantization to compress the speech signal. The LPC analysis is used to estimate the spectral envelope of the speech signal, which represents the shape of the vocal tract. The codebook quantization is used to quantize the LPC parameters and residual signal.

The LPC analysis uses a set of predictor coefficients to model the spectral envelope of the speech signal. These coefficients are estimated using an auto-regressive (AR) model that represents the relationship between the current sample and its past values. The AR model is estimated using a linear prediction algorithm that minimizes the prediction error between the predicted and actual samples.

The codebook quantization is used to quantize the LPC parameters and residual signal. The codebook is a pre-defined set of vectors that are used to represent the LPC parameters and residual signal. The closest vector in the codebook is selected as the quantized value.

The EVRC codec also includes a number of additional features to improve speech quality and reduce background noise. These features include:

• Comfort noise generation: This feature adds artificial noise to the signal during periods of silence to reduce the perceived background noise.
• Voice activity detection: This feature detects periods of speech and adjusts the bit rate accordingly to conserve bandwidth.
• Error detection and correction: This feature detects and corrects errors in the transmitted signal to improve speech quality.
• Frame erasure concealment: This feature fills in missing speech frames due to transmission errors to improve speech quality.

Overall, the EVRC codec provides a high-quality speech compression algorithm for CDMA digital cellular systems. Its variable bit rate and adaptive features make it well-suited for wireless communications and it has become a widely used standard in the United States. The EVRC codec has undergone several revisions since its initial release in 1995. In 1999, an improved version of the codec called EVRC-B (Enhanced Variable Rate Codec - B) was released. EVRC-B uses a wider bandwidth than the original EVRC codec, which allows it to provide higher quality speech at lower bit rates. EVRC-B is also capable of supporting stereo audio and is used in multimedia applications such as video conferencing and streaming.

In addition to EVRC and EVRC-B, there are several other speech coding algorithms that are used in digital cellular systems, including AMR (Adaptive Multi-Rate), GSM (Global System for Mobile Communications) and G.729. Each algorithm has its own strengths and weaknesses, and the choice of algorithm depends on the specific requirements of the system.

One of the main advantages of the EVRC codec is its ability to provide high-quality speech at low bit rates. This is important in wireless communications, where bandwidth is limited and the cost of transmission is a significant factor. By using a variable bit rate and adaptive features such as voice activity detection and comfort noise generation, the codec is able to provide high-quality speech while minimizing the amount of data that needs to be transmitted.

Another advantage of the EVRC codec is its ability to operate in different modes, depending on the complexity of the input signal. This allows the codec to adapt to different types of speech signals and provide optimal performance in a wide range of conditions.

One of the main disadvantages of the EVRC codec is its patent status. Qualcomm Inc. owns several patents related to the EVRC codec, which means that other companies must pay licensing fees to use the codec in their products. This has led to criticism from some industry groups, who argue that the patents create a barrier to competition and innovation.

In conclusion, the Enhanced Variable Rate Codec (EVRC) is a speech coding algorithm that was developed by Qualcomm Inc. for use in CDMA digital cellular systems. The codec uses a combination of linear predictive coding (LPC) and codebook quantization to achieve high-quality speech at low bit rates. Its variable bit rate and adaptive features make it well-suited for wireless communications, and it has become a widely used standard in the United States. While the patent status of the codec has been a source of controversy, its ability to provide high-quality speech compression has made it an important technology in the telecommunications industry.