Apple's Sept. 9 Event: Expect iPhone 6s, Siri UpgradesApple's Sept. 9 Event: Expect iPhone 6s, Siri Upgrades

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Apple's tradition of upgrading its flagship smartphone -- the iPhone -- every September appears to be continuing apace, with the company sending out digital invitations for a special event to be held on Sept. 9 in San Francisco.

The event, which will be held at the Bill Graham Civic Auditorium in the city's downtown, will likely show off new features for the latest iteration of the iPhone -- the iPhone 6s -- which is expected to include Apple's Force Touch technology and improvements to digital concierge Siri.

The most anticipated feature on the updated phones is Force Touch, a technology that detects the force of a tap on a touchscreen and responds differently to variations in pressure.

Force Touch first appeared earlier this year on Apple's new MacBook computers integrated into the notebook's trackpad.

The sensory capabilities of the Force Touch trackpad allow users to tell the MacBook what they want it to do based on subtle differences in the amount of pressure they apply.

Force Touch is also a feature of the Apple Watch, where it uses tiny electrodes around the flexible Retina display to distinguish between a light tap and a deep press, and then trigger access to a range of contextually specific controls.

With Force Touch, a firm press on the screen brings up additional controls in apps like Messages, Music, and Calendar. The feature also lets users select different watch faces, pause or end a workout, and search an address in Maps.

Among the other rumored upgrades to the iPhone 6s are a faster A9 processor, an improved 12-megapixel camera, and 7000 series aluminum, which is more durable, and also used on the Apple Watch.

In typical Apple fashion, the invitations provide little information except for the cryptic tagline, "Hey Siri, give us a hint."

In addition to updates for the iPhone 6s and 6s Plus, the company may also debut updates to Apple TV, its digital media player.

[Here's our wish list for the iPhone 6s.]

Although Apple has reportedly scrapped plans for a more ambitious evolution of its set-top box, the company, which has not updated Apple TV since 2012, is rumored to be announcing the inclusion of Siri.

Difficulties in negotiations between Apple and television content providers such as CBS and 21st Century Fox have resulted in the company pushing back the launch of its Live TV service into 2016, instead of next month, according to a recent report in Bloomberg Business, which cited unnamed sources familiar with Apple's plans.

With sales of the iPhone, Apple's main revenue generator, expected to slow as the global smartphone market approaches the saturation point, the company is looking toward streaming services to keep company growth consistent.