HR (Half-rate)

Last updated on Apr 15, 2023

HR (Half-Rate) is a type of voice codec used in mobile telephony to compress audio signals and enable efficient use of radio resources. The HR codec was developed in the late 1990s as part of the Global System for Mobile Communications (GSM) standard and has since been widely adopted by mobile network operators around the world.

In this essay, we will discuss the technical aspects of the HR codec, its benefits, and drawbacks, and its relevance in the modern mobile telephony landscape.

Technical Aspects of HR Codec

The HR codec uses a lossy compression algorithm to reduce the bit rate required to transmit a voice signal over a mobile network. The original GSM codec used a bit rate of 13 kbps, while the HR codec reduces this to 6.5 kbps. This reduction in bit rate enables more efficient use of radio resources, allowing more calls to be handled within a given frequency band.

The HR codec uses a combination of Linear Predictive Coding (LPC) and Code-Excited Linear Prediction (CELP) techniques to compress the audio signal. The LPC technique uses a mathematical model to predict the next sample in the audio signal based on the previous samples. The difference between the predicted and actual sample is then encoded and transmitted over the network. The CELP technique uses a codebook of pre-defined excitation signals to generate the audio signal, which is then encoded and transmitted.

The HR codec also employs a number of other techniques to reduce the bit rate and improve the quality of the compressed audio signal. These include:

Comfort Noise Generation (CNG): To ensure that the listener is not subjected to complete silence when the speaker is not talking, the HR codec generates artificial background noise known as Comfort Noise. This ensures that the listener is aware that the call is still active and helps to reduce the perception of a drop in call quality.
Error Concealment: In the event that packets of data are lost or corrupted during transmission, the HR codec employs error concealment techniques to fill in the gaps in the audio signal. These techniques may involve interpolating missing samples or synthesizing replacement samples based on the surrounding data.

Benefits and Drawbacks of HR Codec

The primary benefit of the HR codec is its ability to reduce the bit rate required to transmit a voice signal over a mobile network. This enables more efficient use of radio resources and allows more calls to be handled within a given frequency band. This is particularly important in areas of high population density, where radio spectrum is limited, and mobile network operators need to maximize the number of calls that can be handled.

Another benefit of the HR codec is its low complexity, which makes it easy to implement in mobile devices and network infrastructure. The low complexity also means that the codec requires minimal processing power, which is important for mobile devices that are limited in battery life.

However, the HR codec also has some drawbacks. One of the main drawbacks is the lower audio quality compared to other codecs that use higher bit rates. The reduced bit rate and lossy compression algorithm can result in a loss of detail and clarity in the audio signal. This can be particularly noticeable in areas with poor network coverage or when the signal is affected by interference or background noise.

Another drawback of the HR codec is its limited support for advanced features such as noise cancellation and echo suppression. These features are important for ensuring clear audio quality, particularly in noisy environments or when using speakerphone mode. However, due to the limited bit rate of the HR codec, these features are not always supported.

Relevance of HR Codec in Modern Mobile Telephony

While the HR codec was originally developed in the late 1990s, it is still widely used in mobile telephony today. This is because the HR codec provides a good balance between audio quality and efficiency, making it a popular choice for mobile network operators in areas where radio spectrum is limited or where mobile devices have limited processing power.

However, with the advent of 4G and 5G networks, there has been a shift towards using higher bit rate codecs such as Enhanced Voice Services (EVS) and Adaptive Multi-Rate Wideband (AMR-WB). These codecs provide higher audio quality and support for advanced features such as noise cancellation and echo suppression. They are also better suited to handling high-definition voice and video calls, which are becoming increasingly popular with the rise of video conferencing and remote working.

Despite the rise of higher bit rate codecs, the HR codec still has a role to play in modern mobile telephony. It is still widely used in areas where radio spectrum is limited, such as rural areas or in developing countries where mobile network infrastructure is less developed. The low complexity and low processing power requirements of the HR codec also make it well-suited for use in low-end mobile devices, such as feature phones, which are still popular in many parts of the world.

In addition, the HR codec has also been adapted for use in other applications, such as voice over IP (VoIP) and internet telephony. The low bit rate and low complexity of the codec make it well-suited for use in low-bandwidth networks or when there are constraints on processing power.

Conclusion

In summary, the HR codec is a type of voice codec used in mobile telephony to compress audio signals and enable efficient use of radio resources. It uses a lossy compression algorithm to reduce the bit rate required to transmit a voice signal over a mobile network and employs a combination of Linear Predictive Coding (LPC) and Code-Excited Linear Prediction (CELP) techniques to compress the audio signal.

The HR codec provides a good balance between audio quality and efficiency, making it a popular choice for mobile network operators in areas where radio spectrum is limited or where mobile devices have limited processing power. However, with the advent of 4G and 5G networks, there has been a shift towards using higher bit rate codecs such as EVS and AMR-WB, which provide higher audio quality and support for advanced features.