Customizing CPA and Tone Detection

There are a few custom settings that can be applied to both the Call Progress Analysis (CPA) and Answering Machine tone Detection (AMD) algorithms in order to change their behavior. These are controlled using special "CPA" and "AMD" grammars, which signal that these features are needed in the current session or request.

Prior to version 19.1, we provided builtin grammars for both CPA and AMD, however moving forward, we now recommend using variants of the updated grammars we provide and describe in our Grammars in CPA and AMD article. In the majority of cases, you can use these sample grammars directly, or you can make your own copy of them and make your own custom changes. These grammars can be loaded and activated much like other ASR grammars, allowing platforms without any knowledge of either CPA or AMD to use these advanced features seamlessly.

Enabling or Disabling Individual Tone Detection

Within tone detection, there are three different classes of tones that can be detected:

• answering machine tones (AMD)
• fax tones (FAX)
  
• Special Information Tones (SIT)
• or busy tones (BUSY)
  

A tone detection grammar can enable or disable the detection of any of these three types of tones: 

Simply set your AMD_CUSTOM_ENABLE, FAX_CUSTOM_ENABLE, SIT_CUSTOM_ENABLE or BUSY_CUSTOM_ENABLE entries in the grammar to "true" or "false" to turn that type of tone detection on or off.

Changing CPA Timeouts

CPA has four possible results:

• human residence,
  
• human business,
  
• unknown speech (likely machine),
  
• or unknown silence (also likely a machine).
  

CPA returns one of these based on how long it detects human speech (for the first three) or if no speech (silence) is detected. Three timeout settings control this that are specified in the meta tags in the CPA grammar: 

These setting values are all represented in milliseconds (i.e. 1800ms or 1.8 seconds is the default value for CPA_HUMAN_RESIDENCE_TIME).  

If speech is detected that lasts for less than the value of CPA_HUMAN_RESIDENCE_TIME (default 1800 milliseconds), then the HUMAN RESIDENCE prediction result is returned. This is for short utterances typical of residential or mobile phones when answered by a human in a non-business setting (i.e. "Hello this is John")

If the length of speech detected  is greater than CPA_HUMAN_RESIDENCE_TIME, but less than CPA_HUMAN_BUSINESS_TIME, then the HUMAN BUSINESS prediction result is returned. This represents typically longer utterances for humans answering a business line (i.e. "This is LumenVox, how may I direct your call today")

If the speech is longer than CPA_HUMAN_BUSINESS_TIME, then the UNKNOWN SPEECH prediction result is returned. This may be a recorded message or some other automated playback, so is designated as UNKNOWN SPEECH. This is often handled as though it were a machine in call flows. There is no timeout for unknown speech since it is anything longer than human business. Note that once speech had been detected that exceeds the amount of CPA_HUMAN_BUSINESS_TIME, the UNKNOWN SPEECH result will be returned (it does NOT wait for all of the speech to end), allowing your call flow to decide how it would like to proceed.

Note that if CPA_HUMAN_RESIDENCE_TIME and CPA_HUMAN_BUSINESS_TIME are set to the same value, then HUMAN BUSINESS will not be returned. Duration of speech will either be less than these two settings, in which case HUMAN RESIDENCE will be returned, or above these settings, in which case, UNKNOWN SPEECH will be returned. This configuration may be useful for applications that do not wish to distinguish between Residence and Business classifications, and instead only wish to process human versus machine responses. Note that similar behavior can be achieved by setting the appropriate Input Text and Semantic Interpretation values for both of these classifications in the grammar, providing developers maximum flexibility.

UNKNOWN_SILENCE_TIMEOUT controls how long CPA will wait for the start of speech to be detected before returning UNKNOWN SILENCE. Note that once start of human speech has been detected (prior to CPA_UNKNOWN_SILENCE_TIMEOUT), one of the other results will be returned. UNKNOWN SILENCE will only be returned if there is no detected human speech before this timeout is reached.

Also worth noting is the END_OF_SPEECH_TIMEOUT setting (maximum duration of speech after human speech is first detected), which should not be confused with the VAD_EOS_DELAY setting (amount of silence following speech) described below, should also never be reached when working with CPA interactions. Another change introduced in version 19.1 was to force the value of this setting to be 10,000 ms (10 seconds) greater than the largest CPA time specified to ensure it is never reached. Only users working with earlier versions need to apply this setting. All CPA users are encouraged to upgrade to the latest release in order to benefit from some significant performance improvements.

The default values for these CPA timing settings are applied in the client_property.confconfiguration file. Any values (such as those shown above) detectedwithin the grammar will override any defaults specified in theconfiguration file.

CPA_MAX_TIME_FROM_CONNECT

One of the most significant changes to CPA was introduced in version 19.1 is the CPA_MAX_TIME_FROM_CONNECT setting. Be sure to read a more complete description of this setting and how it can be used in our dedicated CPA_MAX_TIME_FROM_CONNECT article.

Basically, this setting overrides and automatically scales the above CPA timing values based on the maximum amount of time your application is willing to wait for a response. This can be used in most applications and has the benefit of being a single setting to apply, which is therefore much less complex.

Modifying the End-Of-Speech Delay

The Voice Activity Detection End-Of-Speech setting (also known as VAD_EOS_DELAY) is used with CPA to determine how much silence should be allowed after start of human speech has been detected to trigger processing of the audio (and generating a response).  A larger value will essentially allow longer pauses between words, while shorter values may trigger end of speech too soon if someone is merely pausing between words.

This value defaults to 1200 ms and should be configured using the CPA grammar file. 

To change the value add the following line to your CPA grammar, placing the desired setting in milliseconds in the content attribute (900 ms in the example).

This value should only be modified with an understanding of the trade-offs, setting it too low will cause more false positive “HUMAN” responses (possibly misclassifying the prediction).  Setting too high of a value will cause the CPA responses to take longer to return.

By way of a simple guide for this, we have found the default value of 1200ms works best in most situations, however implementations that are more sensitive to machine-to-human misclassifications, as may be the case in many call centers, setting a slightly higher value of perhaps 1500ms would improve this accuracy, at the cost of a small amount of additional delay between when the person answering the call stops speaking, and when the system receives a CPA response. In the example here, the difference between 1200ms and 1500ms would mean an additional 300ms of delay (or observed silence) to the person answering the call. Application developers should consider what is an acceptable amount of delay that they wish humans to encounter and understand this trade-off against CPA accuracy.