It’s also necessary that neither the recording nor the playback speed changes over time.Ī phonographic LP is mastered with the intention that it will be played back at a rotational speed of 33 1/3 RPM (Revolutions Per Minute) or 45 RPM, depending on the disc. For example, a film that was recorded at 24 frames (or photos) per second (FPS) must also be played at 24 FPS to avoid objects and persons moving too slowly or too quickly. Rotational speedĮvery recording / playback system, whether for audio or for video signals, is based on the fundamental principle that the recording and the playback happen at the same rate. This detail is also worth noting, since it shows that the rumble value is a relative and not an absolute output level.

If the standard being used for the rumble measurement is the DIN 45 539 specification, then the resulting value is stated as the level difference between the measured filtered noise and a the standard output level, equivalent to the output when playing a 1 kHz tone with a lateral modulation velocity of 70.7 mm/sec. The magnitude responses for the “Unweighted” (black) and “Weighted” filters for rumble measurements, specified in DIN 45 539 Only the high-pass filter specifications are different for the two cases.

Notice that the low-pass filter is the same for both the Weighted and the Unweighted filters. The specifications allow for some deviation of ☑ dB (except at 315 Hz). Note that I’ve only plotted the target response. The characteristics of the filters are defined in internal standards such as DIN 45 539 (or IEC98-1964), shown below. However, that output signal is first filtered to ensure that the level detection is not influenced by higher-frequency problems that may exist. These low-frequency signals are grouped together under the heading of rumble.Ī rumble measurement is performed by playing a disc that has no signal on it, and measuring the output signal’s level. These can include things like mechanical vibrations of the entire turntable transmitted through the table from the floor, vibrations in the system caused by the motor or imbalances in the moving parts, warped discs which cause a vertical movement of the stylus, and so on. This unwanted extraneous movement, and therefore signal, is usually the result of very low-frequency vibrations that come from various sources. This means that any additional movement can (and probably will) be converted to an audio signal that is added to the recording.

Therefore, in order for it to work at all, the system must be sensitive to very small movements in general. As was discussed above, in the specific case of a turntable, the audio signal is initially generated by very small movements of the stylus in the record groove. In practice, however, every audio device adds signals to the output for various reasons. In theory, an audio playback device only outputs the audio signal that is on the recording without any extra contributions. frequency, whereas the frequency response would include both level and phase information. The difference is that a magnitude response only shows output level vs. * This is the correct term for what is typically called the “frequency response”. This is to avoid over-modulation in the low end, and burning out the cutter head during mastering in the high end. Usually, the swept sine signal has a constant amplitude in the low frequency bands (typically, below 1 kHz) and a constant modulation velocity in the high frequencies. It’s also worth noting that a typical standard test disc (JVC TRS-1007 is a good example) will not have a constant output level, which you might expect if you’re used to measuring other audio devices. Consequently, the measurement includes all components in the audio path from the stylus tip, through the RIAA preamplifier (if one is built into the turntable), to the line-level outputs.īecause all of these components are in the signal path, there is no way of knowing immediately whether deviations from the expected response are caused by the stylus, the preamplifier, or something else in the chain. The output level of this signal is recorded at the output of the device, and the level is analysed to determine how much it differs from the expected output. Usually, the magnitude response is measured from a standard test disc with a sine wave sweep ranging from at least 20 Hz to at least 20 kHz. In a turntable, this can be measured in different ways. The magnitude response* of any audio device is a measure of how much its output level deviates from the expected level at different frequencies.

A turntable at rotational speed 33 rpm