BACKGROUND
A mobile phone is used in connection with a vehicle’s HFT using short-range wireless connection (such as Bluetooth®). Here, the mobile phone acts as an “audio gateway” and should provide fully transparent voice transmission in uplink and downlink. The HFT requests the mobile phone to disable its internal signal processing using the control commands (AT-commands) so that the signal processing is solely provided by HFT.
This avoids cascaded signal processing occurring in both the HFT and the mobile phone, the common cause of deteriorated voice quality.
USER EXPECTATIONS
(audio samples of
the driver's voice from the driving car at a speed of around 130 km/h)
Good quality (transparent phone)
Bad quality (noise reduction not disabled, signal amplification)
Bad quality
(noise reduction and echo cancellation not disabled)
COMMON PRINCIPLES
In general, the chain of communication between a mobile phone and a vehicle-mounted HFT is based on the different components which play important roles in establishing a voice connection. Voice is transmitted through some of these components in two transmission directions (Fig.1):
- Receiving direction - Mobile phone -> Hands-free terminal -> Audio play back system
- Sending direction - Microphone -> Hands-free terminal -> Mobile phone
Fig.1 Principle block diagram and definition of transmission directions
Note: the connection between hands-free telephone system and mobile phone is today typically realized via Bluetooth®
In accordance with Hands-Free Profile V1.6 there are some internal signal-processing blocks (such as echo cancellation and noise reduction) which have to be disabled in the mobile phone if an HFT requests so. This is necessary in order to avoid cascaded algorithms which may significantly impair conversational quality either in receiving direction in the vehicle or in sending direction, i.e. at the far-end side.
Fig.2 Cascaded signal processing
The hands-free algorithms provide the echo cancellation functionality (“EC”), the additional processing to suppress residual echo components (echo suppression “ES”), noise reduction algorithm (“NR”) and gain adjustment including possible automatic gain control (“AGC”).
Today’s mobile phones provide the same kind of algorithms (see Fig.2) which may lead to cascaded signal processing if they are not bypassed or disabled. This should always be avoided as the components of the hands-free telephone system are already optimized for the acoustic environment in each vehicle.
TYPICAL ISSUES
Experience shows that cascaded algorithms lead to degradations in conversational quality:
- Additional echo cancellation and echo suppression significantly hamper the very important double-talk performance. The echo suppression unit (“ES” in figure 2) typically introduces attenuation in the microphone path under single-talk conditions if it is only the far-end subscriber talking and their voice is played back via the loudspeakers in the car. In this situation, the driver hears the far-end talker's voice in the car. The implemented signal processing (EC, ES) suppresses the echo, which would be audible and annoying for the person speaking at the far-end side.
If the driver interacts (both persons talk at the same time, designated as “double-talk”) any attenuation in the microphone path, introduced by the echo suppression unit, needs to be quickly removed. Cascaded echo suppression units hamper the conversation (in case where they are not disabled in the mobile phone). The driver’s voice may be significantly attenuated or even partly suppressed (speech gaps in the driver’s voice - “chopped” speech), which is very annoying for the far-end subscriber. - On the other hand, cascaded noise reduction algorithms (“NR” in Figure 2) degrade speech transmission quality, especially if the driver is talking from the driving car. In this case, the driver's voice is transmitted together with background noise from the vehicle. The noise reduction algorithm should lower the transmitted background noise without impairing the quality of the driver’s voice. However, due to technical limitations, quality degradation does occur. The driver’s voice may sound artificial, unnatural and “metallic”, perhaps also disturbed by other artifacts known as “musical tones”. This is audible and annoying for the far-end conversational partner. The degradation is even worse in such a connection if two cascaded noise reduction algorithms are active.
- Other cascaded signal processing, such as additional gain or automatic gain control introduced by the mobile phone may lead to signal saturation either in the receiving direction (distorting the sound of the voice received in the car) or in the microphone path (distorted the driver’s voice audible at the far-end side).
