AUDIO SIGNAL PROCESSING APPROACHES ON SOUND RECORDINGS

At BQ we try to improve every time we launch a new product to the market. Camera, battery or performance are very important nowadays, but also audio. And I am not referring only to playback which is so flashy, but also to recording skills.

We use this recording feature every time we send a voice memo using Whatsapp, audio is recorded in mono, or when we record a video, which is captured in stereo.

In this article I am going to focus on stereo recording and its signal processing techniques but same key points could be applied also for mono recording.

Recording Scenarios

We can distinguish between two different recording scenarios depending on Sound Pressure Level (SPL):

• High SPL scenario: recordings during concerts or in nightclubs. These recordings are characterised by high audio level which could cause so much distortion.
• Low SPL scenario: recordings in silence rooms such as libraries. These are characterised by low audio level which have to be amplified causing high noise floor.

These two scenarios will determine our recording signal processing strategy.

Common Recording Tuning

Our complete recording processing chain is composed by different modules, each module has a specific role. They are set in a specific order as the following image shows.

Recording tuning chain

• Analog Gain: it could be set to different values (0, 6 ,12, 18 or 24 dB). We have set it to 0dB and applied all the necessary amplification in the digital part which gives you more accuracy.
• Analog to Digital Converter (ADC): one dedicated converter for each mic.
• High Pass Filter (HPF) IIR: used to attenuate low frequency components such as wind. Its cutoff frequency is set to 50Hz.
• Digital Gain: it could be set from -72.5 to 18dB in 0.5dB steps. We have set it to 11dB in order to avoid saturation before the compressor.
• Infinite Impulse Response (IIR) Filter: used to make flat mics frequency response. This is necessary due to some mics have a peak level in high frequencies, so it is used to attenuate it.
• Dynamic Range Control (DRC): used to characterise type of recording we want to achieve.

For now on, we are going to focus exclusively in DRC which will determine the behaviour in the different scenarios. I am going to explain how this module works, and different types of processing we can do depending on the scenario.

Dynamic Range Control

This module applies different gains depending on input signal level. It is formed by four parts as you can see in the next image.

DRC sample

• Makeup Gain: it is the total gain applied for middle level signals.
• Compressor: it acts when the signal is too high and needs to be less amplified or even attenuated.
• Noise gate: it acts when the signal is too low, in most cases it is noise, and needs to be less amplified.
• Limiter: it acts only when compressor can’t catch some specific audio sample. It is much more aggressive.

Once we have explained how a DRC works, we are going to described how to adjust it for three scenarios and the difference between them.

BQ Style

We are going to start describing main DRC parameters used in our final DRC. We want to achieve following behaviour:

• Makeup gain set to 18dB this will cause enough makeup gain to amplify low and middle levels.
• Soft compression from -34dBFS will avoid saturation in high SPL scenarios. For a 0dBFS level an attenuation of 5dBFS is applied.
• Soft noise gate from -53dBFS will avoid noise amplification in silent scenarios.
• Limiter is set to -4dBFS, and after it amplifies 3dB.

Loudness Optimized

The main purpose of this processing is to obtain more level so it is perfect for low SPL scenarios. Comparing with processing used changes to apply are:

• Makeup gain set to 24dB, which is the maximum level.
• Harder compression from -34dBFS. For a 0dBFS level same attenuation of 5dBFS is applied.
• Noise gate starts from -63dBFS. This will amplify very low sounds.
• Same limiter.

These changes will cause more noise presence and more low signal amplification. Also, more saturation in high SPL scenarios will be present due to its harder compressor.

Idle Noise & Distortion Optimised

The main purpose of this approach is to avoid idle noise and saturation so it is very convenient for high SPL scenarios. Comparing with tuning used changes to apply are:

• Makeup gain set to 8dB.
• Soft compression from -19dBFS. For a 0dBFS level same attenuation of 5dBFS is applied.
• Soft noise gate from -72dBFS with more drop. This will amplify very low sounds and attenuate silent scenarios.
• Same limiter.

Sacrificing so much level will cause less noise presence and more natural transitions between noise gate and amplification zone. Also, less saturation will be present.

A comparison between these three DRCs can be seen in the next images.

DRC Comparison: left one is BQ Style, center is Loudness Optimized and right is Idle Noise & Distortion Optimised

Recording Audio Tests

In our laboratory we do several tests to analyse and validate the recording performance. These tests are always done in a stable environment within an anechoic chamber. In this article we are going to show three of them and each result of the different approaches so you can get your own conclusions.

The first one is a woman talking at a very low volume, low SPL scenario. We recommend to use headphones for these listening tests setting volume to a high level.

Note: all the recordings were done with the smartphones in portrait mode. This is the reason why the right channel has more level than left one.

The second and third tests are done with the same song. During the second one, it was played at a middle volume. So we recommend to decrease a little the volume of your headphones.

In the third one the volume is set to a level similar to a concert scenario, so we recommend to decrease even more the volume of your headphones.

Which is your favourite processing policy? What would you change? We are willing to know your opinion!