Nokia RRT (Rich Recording Technology) Audio Recording Technology

    We continue to acquaint you with the innovations used in smartphones of the Lumia line. Today we will talk about technology that allows you to record the most important moments on video with perfect sound quality.

    Most microphones have a certain dynamic range - some are sensitive to quiet sounds, while others can perceive very loud ones.
    It is quite difficult to create a microphone that is equally sensitive to sounds of different volume levels, while being relatively compact: the size of the components of a mobile phone is usually limited to 3-4 millimeters.

    To begin with, we will understand how sounds are perceived by the human ear and microphone. The maximum sound pressure level we can tolerate is 140 dB. Noise at a rock concert, sporting event or a noisy party may well reach such a magnitude comparable to the sound of a jet plane taking off at a distance of 25 meters. This is a critical value, and exposure to sound at such a volume can damage your hearing.

    A comparison of typical sources of sound pressure and their level: from a whisper to the sound of a jet airplane taking off.

    Visitors to such noisy events are faced with the inability to record a video with sound without distortion. Microphones installed in ordinary phones operate in the dynamic range up to 87 dB. As a result, when recording video, the audio track is replete with noise if the noise during the recording was too loud. Usually, filtering the recorded sound is used to correct the situation, but this cuts off the low frequencies.

    Software package
    In order for you to enjoy the clear sound of the video shot on Nokia Lumia, we have developed a special Rich Recording Technology (RRT). It consists of both software and hardware solutions aimed at an ideal result. Dynamic Recording Range for Nokia Rich Recording technology is 107 dB (33 to 140 dB SPL)

    Video recording scheme for Nokia Lumia.

    The sound is captured by a microphone, then the data is processed and encoded by the audio stream compression plugin. Video is recorded by the camera module, processed and transcoded. High quality audio recording is provided by an AAC stereo encoder with a sampling frequency of 48 kHz and a bit rate of 192 kbps. HQ video is recorded using an AVC encoder (MPEG-4 Part10 AVC) at 30 frames per second at a resolution of 1980x1080 p and a bit rate of 20 Mbps. Audio and video streams are combined into one container file (.mp4) and saved in a finished file.

    In addition, we have developed a package of sound recording algorithms for improving quality, which includes suppressing wind noise, camera autofocus sound, microphone equalizer and automatic gain control. The package also contains an algorithm for the HAAC microphone and provides an interface and tools for easily adjusting its sensitivity.

    Thanks to these developments, Nokia Rich Recording technology ensures that loud sounds remain without distinguishable noises, while quiet sounds remain discernible.

    Hardware component
    The key hardware component of this technology is the HAAC (High Amplitude Audio Capture) microphone, which is installed in most models of the Lumia line. It has two different sensitivity levels and, accordingly, two channels for recording sound. After capture, the tracks are combined, and the final recording has a wide dynamic range. Due to its high sensitivity, the HAAC microphone perfectly captures very loud and very quiet sounds, and the audio track plays without distortion at any volume.

    Graphically, it looks like this:

    Basic HAAC microphone functionality.

    Device placement
    A great influence on the sound quality has the placement of microphones in the device. We try to place stereo microphones as far as possible from each other and from sources of mechanical noise and vibration in order to avoid unnecessary distortions.

    Comparison of technologies
    Thanks to the extended range of HAAC microphones used, it is possible to record sound with a volume of up to 140 dB without distortion. This means that Nokia Rich Recording technology allows you to record four times louder sound than traditionally used digital microphones.

    As a result of the need to adjust the frequency response to reduce noise, the RRT can provide high quality audio over the entire audible range.

    For your reference: HAAC white paper (.pdf, eng., ~ 146 kB)

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