dBm0

<h2 id="comparison-to-dbfs">Comparison to dBFS</h2>
<p>Digital signals in the abstract digital realm do not necessarily inherently represent any type of measurable physical unit. They are not necessarily relative to any specific reference power level, and thus they need not be expressed as dBm0. But the <a href="/facts/History_of_the_telephone/QthUyv2M">early pioneers of telephonometry</a> gave us the pseudo-digital unit of dBm0, which persists. 
</p><p>A more commonly used unit today for digital signal levels is dB Full Scale or <a href="/facts/DBFS/5Bamuofg">dBFS</a>. The relationship between dBm0 and dBFS is unfortunately ambiguous. It depends how RMS and peak levels in dBFS are defined. 
</p><p>The ambiguity is if a full scale sinusoidal in a digital system is defined to have an RMS level of −3 dBFS RMS or if it should be defined to have a RMS value of 0 dBFS RMS, equal to the dBFS peak value. Today, the interpretation by many companies tend to go towards a definition that a full scale sinusoidal is −3 dBFS RMS and 0 dBFS peak. The only signal that can hold 0 dBFS RMS according to this definition, is a fully saturated square wave. For the relationship between dBm0 and dBFS, this means that +3.14 dBm0 is equivalent to 0 dBFS peak and −3 dBFS RMS. 
</p><p>This also means that the commonly used POI (Point of Interconnect) level of −16 dBm0 can be transformed to −22.14 dBFS RMS in an A-law codec system, or −22.17 dBFS RMS in a μ-law codec system (using the definition of a full scale sinusoidal being −3 dBFS RMS and 0 dBFS peak.
</p><p>Though, there are some companies defining that dBFS RMS equals dBFS peak for sinusoidal signals. Examples are: Qualcomm and Knowles (and other digital <a href="/facts/MEMS/TkhEzDpm">MEMS</a> microphone companies). This gives some consequences when trying to calculate crest factors for speech or noise, because the difference between peak and rms value in analog domain does not correspond to the difference between peak and rms level in digital domain.
</p><p>Other companies like Adobe (software creator of Adobe Audition) and Listen Inc. (software creator of SoundCheck) offer the possibility to choose which dBFS rms definition you want to use in the program.
</p>
<h2 id="notes">Notes</h2>

<h2 id="sources">Sources</h2>
<ul><li><a href="http://portal.etsi.org/stq/ES202020/TIA-912.pdf">ETSI</a></li>
<li><a href="/facts/G.711/kopBvF25">G.711</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>A consequence for the A-law and μ-law codecs of the 0 dBm0 definition is that they have a respective 3.14 dBm0 and 3.17 dBm0 maximum signal level (ratio between the maximum obtainable sine wave amplitude and the specified reference 0 dBm0 sine wave amplitude). <a href="/wiki/A-law" target="_blank">/wiki/A-law</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>0 dBm0 is often replaced by or used instead of digital milliwatt or zero transmission level point. <a href="/wiki/Digital_milliwatt" target="_blank">/wiki/Digital_milliwatt</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>TIA/EIA 810-A, Transmission Requirements for Narrowband Voice over IP and Voice over PCM Digital Wireline Telephones <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
</ol>

dBm0 open-in-new

dBm0