Apple has now sold about 20 million $99 Apple TV devices -- and CEO Tim Cook is confident that the project he once dubbed a "hobby" is a solid competitor to other offerings in the market, including Amazon's Fire TV.

With 20 million Apple TVs, "We've got a pretty large installed base there," Cook said on the company's earnings call Wednesday. "I'm feeling quite good about that business and where it can go."

Cook reiterated on the call that in 2013, the company generated more than $1 billion in revenue from sales of Apple TV hardware and content directly purchased through the devices. "So it didn't feel right to me to refer to something that brought in a billion dollars as a 'hobby,'" he explained.

SEE ALSO: Apple CEO: Apple TV Sales Topped $1 Billion in 2013

Asked his thoughts on HBO's content-licensing pact with Amazon, Cook said he hasn't had a chance to evaluate what that means for Apple. But he noted that Apple TV already offers HBO Go.

"If you look at the content on Apple TV, it is extremely favorable to" what's available on the Amazon Fire TV, he said.

Cook added that Apple is continuing to invest in developing Apple TV, though he didn't shed any light on new features, products or partnerships the Cupertino giant has in the works.

SEE ALSO: Apple-Comcast TV Pact? Don't Hold Your Breath

Asked about future initiatives, Cook said Apple feels "comfortable expanding" into new businesses. But he said "we are not ready to pull the string on the curtain yet." Apple is focused on delivering the highest-quality products and services possible to meet its customers expectations, he said.

"We have some great things we're working on that I'm very, very proud of and very excited about," Cook said. But "we care about every detail... and that takes a bit longer."

Apple has been rumored to be exploring a variety of new businesses and ventures, including a recent report that it is in early talks with Comcast about a streaming-TV service delivered over a dedicated portion of the cable giant's broadband networks to a new Apple set-top.

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