Audio Rating Metric™
Frontline's Audio Rating Metric™ (ARM) is a process for rating audio impairments or differences resulting from problems in Bluetooth audio channels. In the Audio Expert System™ ARM operates only in the reference mode because it produces an objective score based on a set of deterministic measurements of captured reference audio as compared to the original reference audio.
ARM is a real-time process. Individual ARM scores are calculated for each sub-segment of the reference audio. Therefore, the scores are displayed as an overlay on the captured audio waveform providing visual traces of the audio channel quality. For most sub-segments, scores are computed when a change of frequency is detected, and for silent segments, for multi frequency segments, or for level changes. Supported test files can change frequency as often as every 50 milliseconds which equates to ARM's smallest real-time response. However, typical segments resulting from frequency change would occur every 100 milliseconds, as well as a few longer segments.
There are six components to the ARM segment score computations. These components are listed and described in See Audio Rating Metric Score Components. Each metric score is based on the percent change formula
where A and B are the two component values being compared. This computation always results in values between 0.0 and 1.0.
Each component score is multiplied by an associated weighting factor. The exact weight value for each component depends on its relevance with respect to audible degradation, and the sum of the individual weight values must total exactly 1.0. Having prescribed a set of component weights, all audio measured is subjected to the same judgments, and the score outcome of a given audio sample is repeatable.
The maximum ARM score is 5.0 and descends to a minimum of 1.0, in a manner similar to the scoring common to most “Mean Opinion Scores” (MOS). “Errors” found by each of the detection mechanisms effectively “deduct” from the maximum. The implications of the score values with respect to hearer's perceived audio quality are indicated in the following See Audio Rating Metric Scale Interpretation.
|Major Scale Point||Quality||Hearer's Perception of Difference|
|4||Good||Perceptible but not annoying|
The ARM score computation implements a simple linear scaling formula.
ARMfactor = a scaling factor to achieve an ARM range of 1.0 to 5.0.
ARMBaseScore = 5.0.
The percent change formula naturally produces an exponential-like curve for each of the component scores. The ARMfactor inverts it to look appear as in See Audio Rating Metric Score Chart.
The resulting general behavior is that score reduction tends to follow a somewhat logarithmic behavior. Exactly what it takes for any particular component to reduce score from 4, to 3, to 2, etc., depends on the value being measured. But as one example, the number of Frequency Resets that should occur for perfect audio is exactly 1 per frequency change. But, if there are phases changes or other degradations Frequency Resets may increase. Imagining that frequency resets are the only component in the score, the resulting scores for increasing resets would be as shown in See Audio Rating Metric Example: Frequency Reset Impact.
The effective behavior is that the score is fairly sensitive to any initial deduction (impairment), but becomes less sensitive as deductions (impairments) continue to occur. For example, as indicated above, 2 additional unscripted frequency resets (for a total of 3) drops the score by 2 full points to 3.0. But 3 times this, or 6 resets for a total of 7, are required to drop the score by another full point to 2.0.