|Reference / Headsets|
Headsets have been used with a variety of audio devices for decades, often used in a variety of environments. There is no standard for headsets or headset interfaces, partly because the variety of use cases and deployments has created a variety of interfaces. There are interfaces and some standards which have become conventional in certain use cases and environments.
TRS connectors are named after the old telephone jacks which contained a Tip, Ring and Sleeve, these days they are often called phone jacks or audio jack, but they all use the same basic configuration, in different sizes.
Typically the TRS connecter is mated to a headset which contains a speaker and microphone. If the headset is a monaural headset (one ear piece), the TRS connector is used. If the headset is a binaural or stereo headset, a TRRS connecter is used which contains one extra Ring for the second earpiece.
The Tip is normally connected to the microphone. In Vocera B series badges the Tip is connected to the microphone, the Ring is attached to the speaker and Sleeve is the ground.
There may also be a button in the headset, close to the microphone, which can be used to answer or disconnect a call. This is achieved by shorting the connection between the Tip and Sleeve, which is detected by the phone’s headset port and takes action on the event.
The size of the connector has varied over the years, and between different types of equipment. In early cordless phones a 2.5mm connector was very common. As cellular mobile telephones began to appear the 2.5mm connector remained a common size, and many of the early feature phones and indeed some early smart phones would use the 2.5mm connector.
Many of the mobile consumer audio devices would use a 3.5mm TRS connector, and over time the 3.5mm connector became very common on both mobile and desktop consumer audio devices. Eventually being used in consumer computers and laptops, this 3.5mm TRS connector is now the standard audio connector for consumer electronics.
As smartphones emerged and began to blur the lines between cordless telephones and consumer electronics, there was no industry standards body setting the rules for whether to use a 2.5mm or 3.5mm TRS connecter so different manufacturers were free to select the connector that they preferred.
Those equipment manufacturers producing physically small devices tended to favor the 2.5mm connector in order to save space. Early Blackberry, Treo, Cisco, Nokia phones and other vendors who were producing mobile devices quite early on were using the 2.5mm connector. Vocera, as an early manufacturer of the communication devices, also opted for the 2.5mm connecter in order to save space.
As touchscreen smartphones entered the market, they were targeted as both communication devices and entertainment devices, and so inherited the interface of the consumer audio world and were fitted with 3.5mm connectors. Typically these smartphones are designed to play music so they are fitted with a 3.5mm TRRS socket in order to support the stereo earpieces in addition to the microphone.
If an audio device has several options on where to send the sound, the audio device needs to be able to detect when a headset has been connected so that it can direct the sound to the appropriate destination. The ear buds in a headset are normally much closer to a person’s eardrums than the hands free speaker in a device, so the device needs to adjust the audio volume accordingly to avoid harming the person through acoustic shock.
The typical way to detect the insertion of a headset is for the device to measure the impedance between the contacts on the connector. Microphones typically have an impedance between 330Ohms and 1000 Ohms, and speakers typically have an impedance around 4-8 Ohms. If the headset detection in the device measures impedances of these values across the pins of the TRS connector, the device can be fairly confident what kind of headset is connected (TRS or TRRS) and route the audio accordingly.
Vocera Badges contain a 2.5mm TRS socket and headset detection circuitry. The Vocera badges have some extra detection capability to accommodate the use case where a person may wish to listen to a conversation using the headset but use the microphone in the badge. This mode of operation is not normally found in other handheld communication devices and has necessitated some complex headset detection mechanisms.