Voice Transmission
Loudness Rating and Its Predecessor
Historically, on telephone connections, the complaint has been that at the receiving telephone the distant talker’s voice was not loud enough. “Hearing sufficiently well” on a telephone connection is a subjective matter. This is a major element of quality of service (QoS). Various methods have been derived over the years to rate telephone connections regarding customer satisfaction.
The underlying cause of low signal level is loss across the network. Any method to measure “hearing sufficiently well” should incorporate intervening losses on a telephone connection. As we discussed in Chapter 2, losses are conventionally measured in decibels. Thus the unit of measure of “hearing sufficiently well” is the decibel. From the present method of measurement we derive the loudness rating, abbreviated LR. It had several predecessors: reference equivalent and corrected reference equivalent.
Reference Equivalent.
The reference equivalent value, called the overall reference equivalent (ORE), was indicative of how loud a telephone signal is. How loud is a subjective matter. Given a particular voice level, for some listeners it would be satisfactory; for others, unsatisfactory. The ITU in Geneva brought together a group of telephone users to judge telephone loudness. A test installation was set up made up of two standard telephone subsets, namely, a talker’s simulated subscriber loop and a listener’s simulated loop. An adjustable attenuating network was placed between the two simulated loops. The test group, on an individual basis, judged level at the receiving telephone earpiece. At a 6-dB setting of the attenuator or less, calls were judged too loud. Better than 99% of the test population judged calls to be satisfactory with an attenuator setting of 16 dB; 80% rated a call satisfactory with an ORE 36 dB or better, and 33.6% of the test population rated calls with an ORE of 40 dB as unsatisfactory, and so on.
Using a similar test set up, standard telephone sets of different telephone administrations (countries) could be rated. The mouthpiece (transmitter) and earpiece (receiver) were rated separately and given a decibel value. The decibel value was indicative if they worked better or worse than the telephones used in the ITU laboratory. The attenuator setting represented the loss in a particular network connection. To calculate overall reference equivalent (ORE), we summed the three decibel values (i.e., the transmit reference equivalent of the telephone set, the intervening network losses, and the receive reference equivalent of the same type subset). In one CCITT recommendation, 97% of all international calls were recommended to have an ORE of 33 dB or better. It was found that with this 33-dB value, less than 10% of users were unsatisfied with the level of the received speech signal.
Corrected Reference Equivalent
Because difficulties were encountered in the use of reference equivalents, the ORE was replaced by the corrected reference equivalent (CRE) around 1980. The concept and measurement technique of the CRE was essentially the same as RE (reference equivalent), and the decibel remained the measurement unit.
CRE test scores varied somewhat from its RE counterparts. Less than 5 dB (CRE) was too loud; an optimum connection had an RE value of 9 dB and a range from 7 to 11 dB for CRE. For a 30-dB value of CRE, 40% of a test population rated the call excellent, whereas 15% rated it poor or bad.
Loudness Rating
Around 1990 the CCITT replaced corrected reference equivalent with loudness rating. The method recommended to determine loudness rating eliminates the need for subjective determinations of loudness loss in terms of corrected reference equivalent. The concept of overall loudness rating (OLR) is very similar to the ORE concept used with reference equivalent.
Table 3.1 gives opinion results for various values of overall loudness rating (OLR) in decibels. These values are based upon representative laboratory conversation test results for telephone connections in which other characteristics such as circuit noise have little contribution to impairment.
Determination of Loudness Rating
The designation with notations of loudness rating concept for an international connection is given in Figure 3.2. It is assumed that telephone sensitivity, both for the earpiece and microphone, have been measured. Overall loudness rating (OLR) is calculated using the following formula:
The measurement units in Eq. (3.2) are decibels.
The overall loudness rating (OLR) is defined as the loudness loss between the speaking subscriber’s mouth and the listening subscriber’s ear via a telephone connection. The send loudness rating (SLR) is defined as the loudness loss between the speaking subscriber’s mouth and an electrical interface in the network. The receive loudness rating (RLR) is the loudness loss between an electrical interface in the network and the listening subscriber’s ear. The circuit loudness rating (CLR) is the loudness loss between two electrical interfaces in a connection or circuit, with each interface terminated by its nominal impedance.