5.1.6 Test setup for electrical interfaces

26.1323GPPRelease 18Speech and video telephony terminal acoustic test specificationTS

5.1.6.1 Wired analogue connection

UE testing via analogue connection shall be carried out with a universal wired headset interface, which complies with the electrical and physical characteristics described in clause 6 of ITU‑T P.381 [53]. In case the UE is not equipped with this type of socket, but an associated adapter/converter is provided, testing shall be conducted with this additional equipment instead. In case also no associated adapter/converter is provided, but an USB‑C port supporting the Audio Adapter Accessory Mode according to Annex A of [56] is available, testing shall be conducted with a generic analogue adapter (Annex A of [56]). Other implementations of analogue electrical interfaces (wired or wireless) are out of scope.

Figure 15a5b illustrates the setup required for testing analogue electrical interfaces. The electric output impedance of the reference interface of the test equipment shall be in the range of 1 Ω and 10 kΩ. The corresponding electric input impedance shall be 32 Ω +/- 2 Ω. The common ground impedance (between sending and receiving sides) for the test system shall be ≤ 0.05 Ω.

If not specified otherwise, the nominal signal levels are:

– -60 dBV in send direction (input to electrical interface UE), which corresponds to an acoustic level of -4.7 dBPa at the MRP, i.e., a default sensitivity of ~55 dBV/Pa.

– -39 dBV in receive direction (for an electrical interface UE providing stereo/diotic output), for a nominal volume setting (if present).

For the receive direction, it is expected that the output signals of the electrical interface UE are identical or at least very close. If not specified otherwise, all measurements in receive shall be conducted with just one of the two channels. For such measurements, the used channel shall be reported.

For testing echo and double talk scenarios, an artificial feedback of the receive signal into the sending path shall be used. This echo path shall be realized in a digital way, e.g., part of the test system. Analogue realizations, e.g., a stand-alone device, are for further study.

To apply a certain echo loss (in dB), it is typically assumed that the nominal level for send and receive path are identical. For analogue electrical interface, the difference in nominal levels of -21 dB shall be considered in test setups.

For measurements without artificial echo loss, the feedback path is disabled.

NOTE: It is assumed that mainly passive third-party devices are connected via analogue electrical interface to the UE, which do not contain any typical signal processing capabilities (like e.g., echo cancellation or noise reduction). Thus, all tests specified for this interface are comparable to handset UE or headset UE, i.e. they expect that any possible signal processing is applied in the UE.

Figure 15a5b: Test setup for analogue electrical headset interface

5.1.6.2 Digital connection

Figure 15a5c illustrates the setup required for testing digital electrical interfaces of a UE. Such testing is possible with different types of wired and wireless technologies and requires a digital reference interface in the test system. The present document is only applicable to USB and Bluetooth interfaces, other digital interfaces are out of scope.

For some combinations of digital electrical interface and codec type, the overall audio bandwidth may be lower as specified in clause 5.4. In such cases, the electrical interface UE shall be tested according to the overall audio bandwidth.

EXAMPLE: Electrical interface UE is connected via Bluetooth transmission in wideband mode to the test equipment. The telephony part of the UE operates with EVS codec in super-wideband mode. Tests for this combination are conducted according to clause 8 (wideband mode) instead of clause 9 (super-wideband mode).

For testing echo and double talk scenarios, an artificial feedback of the receive signal into the sending path shall be used. This echo path shall be realized as part of the test system. The received and decoded signal from the UE is fed back into the sending direction, in advance to the encoding/protocol/hardware layer. For measurements without artificial echo loss, the feedback path is disabled.

Digital levels for send and receive direction are specified in dBm0, referring to the same definition as for the input/output of the terminal and the system simulator (see clause 5.2).

In contrast to the analogue interface, devices connected to the UE via digital interface may either provide active (includes signal processing for send and/or receive direction) or passive (comparable to analogue headsets, see clause 5.1.6.1) functionalities. Tests are only applicable in case of typical signal processing for telecommunication (e.g., noise reduction, echo cancellation) takes places only in the UE and not in the equipment to be connected. If necessary and if the digital interface and the associated protocols support the exchange of commands/meta-information, the electrical interface UE shall be configured in such away that carries out its own signal processing. Performance tests according to clauses 7-10 are not applicable in case the signal processing in the UE cannot be enabled in any way for the electrical interface.

In some cases, a digital headset with higher computing capabilities may provide and signal its own signal processing when connecting to the electrical interface UE. It is expected that the UE will take the headset’s capabilities into account to avoid possible tandem signal processing. For this purpose, a minimum set of transparency tests are described in Annex G and shall be conducted in advance to the actual testing. The results of the transparency tests shall be reported.

A full set of appropriate transparency tests can be found in Recommendation ITU-T P.383 [55], but further testing beyond Annex G is out of scope for the present document.

Figure 15a5c: Test setup for digital electrical headset interface

The digital reference interface shall comply with the accuracy requirements for test equipment described in clause 5.3.

It is expected that the volume control at the digital electrical interface UE does not have a direct effect on the signal in receive direction. In most cases, the volume setting at the connected equipment with acoustical interface (e.g., a digital headset) is remote-controlled instead.

In consequence, the digital reference interface shall not attenuate or amplify the digitally transmitted signal at the electrical interface in case the volume control at the UE is changed. On the other hand, test methods are only applicable for a single volume setting. Tests shall be conducted with volume control set to maximum at the UE.

NOTE: For sake of simplicity and clarity, this single volume setting is regarded as "nominal volume" in the test descriptions of the following clauses.

Since it is expected that the signal level at the electrical interface UE is independent of the type of equipment connected (e.g., monaural, or binaural headset), test methods related to binaural listening generally do not apply.