# Methodology for Obtaining High-Quality Speech Corpora
## 4 Assessing the Quality of Speech Corpora
### Technical Parameters of Speech Recordings  
The technical parameters of speech recordings include audio file parameters (the bit depth, sampling rate, number of channels, and format--uncompressed/compressed), equipment parameters (the specifications of the microphones used and their setup; studio or non-studio recordings), and the technical quality of the recordings (SNR). In the case of speech, there is no need to record the full range of audible frequencies. For plain old telephone service (POTS) recording, a sampling rate of only 8 kHz is sufficient (higher frequencies, above 4 kHz, are missing in such signals), and a 44.1 kHz sampling rate of compact discs (CDs) covers the full human hearing range of most people. The sampling rate is one of the criteria for assessing sound data quality because it determines the frequency range that can be recorded (half the sampling rate). A sampling rate of at least 16 kHz is recommended, which is a popular choice in speech corpora, especially if the recorded speech is to be used for ASR purposes. The higher the sampling rate, the higher frequencies can be recorded, but files also become larger. The use of a lossless or lossy bit format also determines the quality of the recorded sound. Lossy compression, e.g., mp3, should be avoided, because it degrades the sound spectrum and introduces artifacts to obtain smaller files. The uncompressed.wav format with linear pulse code modulation (PCM) encoding is the most common choice in speech corpora; aiff, au, or.flac formats can also be applied to store audio data for Apple systems, Unix systems, or with lossless compression, respectively. Digital audio data stored using linear PCM represent quantized amplitudes of the audio signal, sampled at uniform intervals, and quantized also at uniform intervals, i.e., linearly. The use of a lossless sound format, or at least avoiding lossy compression, allows high-quality files to be obtained [86]. Information about the sound format, as well as the sampling rate and bit depth, should be provided to corpus users. Assuming that linear PCM is applied to represent digitally recorded sound, the greater the number of bits used, the greater the SNR can be obtained, and an increase of one bit improves the SNR by 6 dB. Generally, the bit depth used in audio recording determines the maximum SNR that can be achieved, and a higher bit depth means a wider dynamic range. Therefore, the bit depth also constitutes a criterion for assessing sound quality. A bit depth of at least 16 bits should be used to avoid high levels of quantization noise. To achieve a high SNR, the recorded speech should be loud enough (but not too loud to avoid overdrive), and background noise should be avoided by assuring a quiet environment. To this end, the speakers should be recorded at a proper distance depending on the microphone selected. A test recording is recommended at the beginning of the recording session. The recording environment is another technical criterion of recorded speech quality. Studio recordings are performed by professional sound engineers, using properly set high-quality microphones in an acoustically treated studio and using professional equipment. Therefore, studio-recorded speech is assessed as high-quality data. Additionally, professional voice actors are often recorded in studios, and in such cases, the speech itself is of high quality. Sound recorded outside of studios can also represent high quality. Recordings made via mobile devices (instead of sound recorders) have also become popular in recent years. Good-quality recordings can be obtained using smartphones if the speech is recorded from a close distance, in a quiet environment, at least without human speech in the background [86], possibly with surfaces absorbing reverberation, and using a lossless sound format of sufficient quality. If possible, an external microphone can be used; if needed, portable equipment can be purchased for recording.