With the advent of wireless headphones, there has been a steadily increasing demand for HD or High Definition audio. People of all ages like the HD sound experience, especially those with age-related hearing degradation. These trends are driving the HD audio support development at all stages of the delivery chain.
High definition audio or high-resolution audio has no strict technical definition. Industry experts use the term to describe audio systems supporting higher data rates than older equipment could handle. Initially, industry experts first used the term to describe digital systems that could handle higher data rates as compared to the Compact Disc format. Now, it applies to wireless headphones that can deliver better audio quality.
Although the industry has improved the recording and distribution of audio with increased data rates and these are available to mobile listeners, wireless headphones were left behind mainly due to Bluetooth limitations. With newer Bluetooth codecs, it is now possible for wireless headphones too to deliver HD quality audio. Therefore, the trend is to improve the audio hardware, especially the drivers.
Digital audio formats are mainly defined by two terms — sample rate and bit depth. When converting from the analog sound, the digital audio samples the signal amplitude and saves each sample as a binary number. The sample rate represents the number of times the system samples the analog signal every second. The binary number size that represents the amplitude is the bit depth.
To accurately capture the information in a sine wave, it is necessary to sample it at least two times per cycle. Therefore, for music, the sample rate must be at least two times the highest frequency in the music. Therefore, if the maximum audio frequency to be reproduced is 20 kHz, the sampling frequency must be at least 40 kHz. Additionally, the ADC will require a very sharp 20 kHz low-pass filter to remove all frequencies above 20 kHz. However, in practice, nothing is perfect. Therefore, experts set the actual sample rate to 44.1 kHz, as this produces a better hearing experience.
The size of the byte that describes the audio sample, or its bit depth, determines the accuracy of each sample when digitized. Each additional bit in the binary word describes the amplitude with twice the original number of values. Alternately, this reduces errors by a factor of two. In the digital music system, this reduces the distortion and quantization noise, allowing each added bit of depth to reduce the noise floor by 6 dB.
Typically, digital systems work on multiples of 8 bits. Therefore, digital audio also uses multiples of 8 bits for its word size. With 8 bits as the word size, the noise floor is only 48 dB below the loudest music and is not very practical. Compact discs use a bit depth of 16, providing a signal-to-noise ratio of 96 dB, which is more reasonable.
At present, it is possible to deliver HD audio to the listener, as it is possible to upgrade all the stages in the delivery chain. Bluetooth codecs have been upgraded, mobile phone capability is better, and music streaming services have improved.