Silence detection: Difference between revisions

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VoIPmonitor is able to detect (currently only for G711a/u) silence in RTP streams. Silence threshold is hardcoded in the code (it is tolerant to a little noise). It must be enabled in voipmonitor.conf
{{DISPLAYTITLE:Audio Analysis: Silence & Clipping Detection}}
[[Category:GUI manual]]


'''This guide explains VoIPmonitor's audio analysis features for G.711 codecs, including silence and clipping detection. These tools diagnose audio quality issues not caused by network problems, such as one-way audio, muted microphones, or faulty hardware.'''


silencedetect = yes
== Overview ==


Standard network metrics (packet loss, jitter) only identify transport-layer problems. Many audio issues originate at endpoints. The Silence & Clipping Detection engine analyzes decoded G.711 audio to identify these problems.


MySQL table cdr must have those columns:
<kroki lang="plantuml">
@startuml
skinparam shadowing false
skinparam defaultFontName Arial


caller_silence, called_silence, caller_silence_end, called_silence_end
rectangle "RTP Stream\n(G.711 audio)" as RTP
caller_clipping_div3, called_clipping_div3
rectangle "Audio Decoder" as Decoder
a_mos_silence_min_mult10, b_mos_silence_min_mult10, _mos_silence_mult10, b_mos_silence_mult10
rectangle "Silence Detection\n(threshold check)" as Silence
rectangle "Clipping Detection\n(max amplitude)" as Clipping
database "CDR Database\n(metrics stored)" as DB
rectangle "MOS Adjustment\n(silence = loss)" as MOS


If any of those columns are missing this feature will be not enabled and you need manually alter cdr table (it can take long time blocking access to GUI and sniffers during the upgrade).
RTP --> Decoder : decode a-law/u-law
Decoder --> Silence : audio samples
Decoder --> Clipping : audio samples
Silence --> DB : silence %, silence_end
Clipping --> DB : clipping count
Silence --> MOS : silence frames
MOS --> DB : adjusted MOS scores
@enduml
</kroki>


'''Use cases:'''
* "Dead air" complaints when network statistics look perfect
* One-way audio (one party not transmitting)
* Audio clipping (clicks or distortion)
* Accurate [[Glossary|MOS]] calculation by penalizing silence periods
* Detecting calls in noisy environments (see [[#Detecting Calls with Background Noise|below]])


Full alter commands: (remove columns from following sql command which you already have in the cdr table). When you run voipmonitor it also prints alter commands which are missing (on syslog and stdout)
== Prerequisites ==


{| class="wikitable"
|-
! Requirement !! Details
|-
| '''Codec''' || G.711 a-law/u-law only. Other codecs are not analyzed.
|-
| '''CPU''' || Audio decoding is CPU-intensive. Monitor system load after enabling.
|-
| '''Database''' || Modern installations (2020+) have required columns. Legacy systems may need schema updates (see below).
|}


ALTER TABLE cdr
== Configuration ==
ADD COLUMN caller_silence tinyint unsigned default NULL,
ADD COLUMN called_silence tinyint unsigned default NULL,
ADD COLUMN caller_silence_end smallint default NULL,
ADD COLUMN called_silence_end smallint default NULL;,
ADD COLUMN caller_clipping_div3 smallint unsigned default NULL,
ADD COLUMN called_clipping_div3 smallint unsigned default NULL,
ADD COLUMN a_mos_silence_min_mult10 tinyint unsigned DEFAULT NULL,
ADD COLUMN b_mos_silence_min_mult10 tinyint unsigned DEFAULT NULL,
ADD COLUMN a_mos_silence_mult10 tinyint unsigned DEFAULT NULL,
ADD COLUMN b_mos_silence_mult10 tinyint unsigned DEFAULT NULL;


When enabled following metrics are produced:
=== Parameters ===


* Number of absolute silence audio frames (with 20ms length) which are treated as packet loss. On the following picture see the holes in the first audio stream - this example is taken from a call where RTP stream is coming without any packet loss or jitter, those audio drops are encoded directly in the RTP audio stream. With silencedetection = yes feature voipmonitor will calculate number of those silence frames and counts them as packet loss and calculates MOS score.  
{| class="wikitable"
|-
! Parameter !! Default !! Description
|-
| <code>silencedetect</code> || no || Enable silence detection and MOS adjustment
|-
| <code>clippingdetect</code> || no || Enable audio clipping detection
|-
| <code>silencethreshold</code> || 512 || Audio level threshold for silence. Lower = stricter detection.
|}


[[File:cdr_spectral_silence_example.png]]
=== Enable in voipmonitor.conf ===
 
<syntaxhighlight lang="ini">
# /etc/voipmonitor.conf
silencedetect = yes
clippingdetect = yes
silencethreshold = 512
</syntaxhighlight>
 
<syntaxhighlight lang="bash">
systemctl restart voipmonitor
</syntaxhighlight>
 
{{Note|1=For legacy installations (pre-2020) that report missing columns on startup, add them via SQL:
<syntaxhighlight lang="sql">
ALTER TABLE cdr
  ADD COLUMN caller_silence tinyint unsigned default NULL,
  ADD COLUMN called_silence tinyint unsigned default NULL,
  ADD COLUMN caller_silence_end smallint default NULL,
  ADD COLUMN called_silence_end smallint default NULL,
  ADD COLUMN caller_clipping_div3 smallint unsigned default NULL,
  ADD COLUMN called_clipping_div3 smallint unsigned default NULL,
  ADD COLUMN a_mos_silence_min_mult10 tinyint unsigned DEFAULT NULL,
  ADD COLUMN b_mos_silence_min_mult10 tinyint unsigned DEFAULT NULL,
  ADD COLUMN a_mos_silence_mult10 tinyint unsigned DEFAULT NULL,
  ADD COLUMN b_mos_silence_mult10 tinyint unsigned DEFAULT NULL;
</syntaxhighlight>
}}
 
== Features & Metrics ==


=== Silence as Packet Loss (MOS Adjustment) ===


The MOS score is shown in the Graph in CDR detail as new MOS value "Sil" - see the RED 2.8 number
Silence frames (e.g., muted mic) are treated as packet loss, allowing accurate [[Glossary|MOS]] calculation for silent calls. In CDR Graph Detail, a new '''"Sil"''' (Silence MOS) score appears.


[[File:cdr_spectral_silence_example.png]]
[[File:cdr_graph_silence_lowmos_example.png]]
[[File:cdr_graph_silence_lowmos_example.png]]


=== Overall Silence Percentage ===
CDR Detail shows total silence percentage for caller and callee streams.


* Another silence feature is overall percentage of silence in each direction of a call which will be shown in cdr detail. Calls with >95% percent of a silence would indicate problem.  
{{Tip|1=>95% silence in one direction strongly indicates one-way audio.}}


[[File:cdr_detail_silence_table.png]]
[[File:cdr_detail_silence_table.png]]


* number of silence seconds from the end of the last RTP packet - this might be the reason why someone hanged up the call because although RTP packets were coming, there was a silence. If value is non zero it will be shown also in CDR summary:
=== Silence at End of Call ===
 
Measures silence duration before hangup. High values may indicate user frustration (couldn't hear other party). Shown in main CDR listing.


[[File:cdr_summary_silence_example.png]]
[[File:cdr_summary_silence_example.png]]


* Number of clipped frames (clipped frame has the highest volume) - perceived as audible clicks - this value is shown in cdr detail table. It can be filtered and used in charts. Must be enabled in voipmonitor.conf
=== Audio Clipping Detection ===
 
Counts clipped frames (signal too loud, hits max amplitude = distortion). Shown in CDR detail; usable for filtering and charts to find problematic endpoints.
 
[[File:charts_silence_values.png]]


clippingdetect = yes
=== MOS During Silence Periods ===


Data series in charts for silence, silence end and clipped frames.
{| class="wikitable"
|-
! Column !! Description
|-
| <code>a_mos_silence_mult10</code> / <code>b_mos_silence_mult10</code> || Average MOS during silence (caller/callee)
|-
| <code>a_mos_silence_min_mult10</code> / <code>b_mos_silence_min_mult10</code> || Minimum MOS during silence (caller/callee)
|}


[[File:charts_silence_values.png]]
{{Note|These metrics only populate when <code>silencedetect=yes</code>. Otherwise, regular MOS is shown.}}
 
== Detecting Calls with Background Noise ==
 
Calls with background noise show '''low silence percentage''' because continuous audio prevents silence detection.
 
{| class="wikitable"
|-
! Environment !! Silence % Range
|-
| Clean/quiet office || 40-70%
|-
| Environmental noise (A/C, distant voices) || 10-39%
|-
| High noise (factory, crowded call center) || 0-9%
|}
 
'''Workflow:'''
# Enable <code>silencedetect=yes</code> with appropriate <code>silencethreshold</code>
# Filter CDR for <code>Caller Silence < 20%</code> OR <code>Called Silence < 20%</code>
# Review flagged calls' audio recordings to confirm noise
# Create saved filters or [[Alerts|alerts]] for ongoing monitoring
 
{{Tip|Use spectral analysis in CDR Graph Detail to distinguish white/brown noise (continuous spectrum) from sporadic noise (intermittent) and genuine speech (natural patterns with pauses).}}
 
== See Also ==
 
* [[Glossary]] - MOS, jitter, packet loss definitions
* [[Comprehensive_Guide_to_VoIP_Voice_Quality]] - Voice quality factors
* [[Call_Detail_Record_-_CDR]] - CDR view and filtering
* [[Charts]] - Creating charts from silence/clipping data
* [[Alerts]] - Setting up alerts for quality thresholds
 
== AI Summary for RAG ==
 
'''Summary:''' VoIPmonitor's Silence and Clipping Detection analyzes G.711 (a-law/u-law) audio to diagnose issues invisible to network metrics. Key features: treats silence as packet loss for accurate MOS calculation, calculates silence percentage (>95% indicates one-way audio), measures end-of-call silence (user frustration indicator), counts clipped frames (distortion), and detects calls with background noise (low silence %). Configuration requires <code>silencedetect=yes</code> and <code>clippingdetect=yes</code> in voipmonitor.conf. Modern installations have required database columns by default.
 
'''Keywords:''' silence detection, clipping, one-way audio, dead air, MOS, G.711, silencedetect, clippingdetect, silencethreshold, mos_silence, background noise, noisy environments, audio analysis, endpoint issues
 
'''Key Questions:'''
* How can I diagnose one-way audio problems?
* Why is a call's MOS score high even though the user reported hearing nothing?
* How does VoIPmonitor detect silence in a call?
* What does the "Sil" (Silence MOS) score mean in the CDR graph?
* How can I find calls with audio clipping?
* How can I filter calls based on background noise?
* What silence percentage indicates a noisy environment?

Latest revision as of 16:47, 8 January 2026


This guide explains VoIPmonitor's audio analysis features for G.711 codecs, including silence and clipping detection. These tools diagnose audio quality issues not caused by network problems, such as one-way audio, muted microphones, or faulty hardware.

Overview

Standard network metrics (packet loss, jitter) only identify transport-layer problems. Many audio issues originate at endpoints. The Silence & Clipping Detection engine analyzes decoded G.711 audio to identify these problems.

Use cases:

  • "Dead air" complaints when network statistics look perfect
  • One-way audio (one party not transmitting)
  • Audio clipping (clicks or distortion)
  • Accurate MOS calculation by penalizing silence periods
  • Detecting calls in noisy environments (see below)

Prerequisites

Requirement Details
Codec G.711 a-law/u-law only. Other codecs are not analyzed.
CPU Audio decoding is CPU-intensive. Monitor system load after enabling.
Database Modern installations (2020+) have required columns. Legacy systems may need schema updates (see below).

Configuration

Parameters

Parameter Default Description
silencedetect no Enable silence detection and MOS adjustment
clippingdetect no Enable audio clipping detection
silencethreshold 512 Audio level threshold for silence. Lower = stricter detection.

Enable in voipmonitor.conf

# /etc/voipmonitor.conf
silencedetect = yes
clippingdetect = yes
silencethreshold = 512

systemctl restart voipmonitor

ℹ️ Note: For legacy installations (pre-2020) that report missing columns on startup, add them via SQL: 

ALTER TABLE cdr
  ADD COLUMN caller_silence tinyint unsigned default NULL,
  ADD COLUMN called_silence tinyint unsigned default NULL,
  ADD COLUMN caller_silence_end smallint default NULL,
  ADD COLUMN called_silence_end smallint default NULL,
  ADD COLUMN caller_clipping_div3 smallint unsigned default NULL,
  ADD COLUMN called_clipping_div3 smallint unsigned default NULL,
  ADD COLUMN a_mos_silence_min_mult10 tinyint unsigned DEFAULT NULL,
  ADD COLUMN b_mos_silence_min_mult10 tinyint unsigned DEFAULT NULL,
  ADD COLUMN a_mos_silence_mult10 tinyint unsigned DEFAULT NULL,
  ADD COLUMN b_mos_silence_mult10 tinyint unsigned DEFAULT NULL;

Features & Metrics

Silence as Packet Loss (MOS Adjustment)

Silence frames (e.g., muted mic) are treated as packet loss, allowing accurate MOS calculation for silent calls. In CDR Graph Detail, a new "Sil" (Silence MOS) score appears.

Overall Silence Percentage

CDR Detail shows total silence percentage for caller and callee streams.

💡 Tip: >95% silence in one direction strongly indicates one-way audio.

Silence at End of Call

Measures silence duration before hangup. High values may indicate user frustration (couldn't hear other party). Shown in main CDR listing.

Audio Clipping Detection

Counts clipped frames (signal too loud, hits max amplitude = distortion). Shown in CDR detail; usable for filtering and charts to find problematic endpoints.

MOS During Silence Periods

Column Description
a_mos_silence_mult10 / b_mos_silence_mult10 Average MOS during silence (caller/callee)
a_mos_silence_min_mult10 / b_mos_silence_min_mult10 Minimum MOS during silence (caller/callee)

ℹ️ Note:

Detecting Calls with Background Noise

Calls with background noise show low silence percentage because continuous audio prevents silence detection.

Environment Silence % Range
Clean/quiet office 40-70%
Environmental noise (A/C, distant voices) 10-39%
High noise (factory, crowded call center) 0-9%

Workflow:

  1. Enable silencedetect=yes with appropriate silencethreshold
  2. Filter CDR for Caller Silence < 20% OR Called Silence < 20%
  3. Review flagged calls' audio recordings to confirm noise
  4. Create saved filters or alerts for ongoing monitoring

💡 Tip: Use spectral analysis in CDR Graph Detail to distinguish white/brown noise (continuous spectrum) from sporadic noise (intermittent) and genuine speech (natural patterns with pauses).

See Also

AI Summary for RAG

Summary: VoIPmonitor's Silence and Clipping Detection analyzes G.711 (a-law/u-law) audio to diagnose issues invisible to network metrics. Key features: treats silence as packet loss for accurate MOS calculation, calculates silence percentage (>95% indicates one-way audio), measures end-of-call silence (user frustration indicator), counts clipped frames (distortion), and detects calls with background noise (low silence %). Configuration requires silencedetect=yes and clippingdetect=yes in voipmonitor.conf. Modern installations have required database columns by default.

Keywords: silence detection, clipping, one-way audio, dead air, MOS, G.711, silencedetect, clippingdetect, silencethreshold, mos_silence, background noise, noisy environments, audio analysis, endpoint issues

Key Questions:

  • How can I diagnose one-way audio problems?
  • Why is a call's MOS score high even though the user reported hearing nothing?
  • How does VoIPmonitor detect silence in a call?
  • What does the "Sil" (Silence MOS) score mean in the CDR graph?
  • How can I find calls with audio clipping?
  • How can I filter calls based on background noise?
  • What silence percentage indicates a noisy environment?
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