Harmonix achieved two unprecedented goals with Rock Band 4, and both required a lot of elbow grease. The studio conceived Rock Band 4 as a platform, designed to support as much of the music that players had previously purchased, and as many of the plastic instruments gathering dust in their closets, as possible.

"People have spent good money on their controllers and it seemed crazy to force folks who already have good working controllers to buy new ones," said Daniel Sussman, project manager on Rock Band 4 at Harmonix, in an email interview with Polygon.

Among the challenges that Harmonix faced on the instrument front, perhaps none loomed larger than the connectivity conundrum. Wireless Rock Band and Guitar Hero controllers required USB dongles on PlayStation 3, but they communicated directly with the Xbox 360 — no extra equipment necessary.

PS3 instruments work seamlessly with Rock Band 4 on PlayStation 4 (as long as you still have those USB doohickeys lying around). But Xbox 360 controllers aren't compatible with an Xbox One, which presented Harmonix with a problem.

The plan

Harmonix co-published Rock Band 4 with Mad Catz, and the latter company handled the development of new controllers for the game — guitar, drum set and microphone. Mad Catz also worked with Harmonix to build the eventual solution for Xbox instrument compatibility. Representatives from both companies said that they didn't want to compromise on ensuring compatibility with previous purchases of songs or instruments.

Delivering backward compatibility for controllers would prove to be a much more complicated process on Xbox One than on PS4. Microsoft used a proprietary wireless protocol for controllers on both the Xbox 360 and the Xbox One, but the two standards are different, making it impossible to use Xbox 360 peripherals with an Xbox One and vice versa.

"We didn't have a lot of options"

"We didn't have a lot of options given the degree to which the chip set and fundamental wireless technology changed between Xbox 360 and Xbox One," Sussman explained.

Asked if this roadblock ever led Harmonix to consider abandoning the Xbox platform and supporting backward compatibility only on PlayStation, Sussman said no, adding that the studio "knew that this was a feature that needed to [...] exist in an all-or-none capacity."

A USB dongle was the only way forward on Xbox. That seems like a simple solution: Develop a small piece of hardware that can communicate wirelessly with Xbox 360 guitar controllers, and plug it into an Xbox One. It turned out to be anything but.

The process

Development on Rock Band 4 began in the last quarter of 2014, said Mad Catz's Richard Neville, senior product development manager, and Simon Bell, technical director, in an email interview. The USB adapter was part of the plan all along, and Mad Catz worked with Microsoft to develop the device. Bell and Neville said Microsoft was "incredibly supportive" during the process, providing resources such as important documentation, old controllers for testing and connections to component suppliers.

Neville and Bell provided details on the time-intensive development of the dongle. Mad Catz's developers created a prototype and hooked it up to a computer in order to capture the inputs from a variety of Xbox 360 instruments. (The games' microphones have always been wired USB devices, so they're platform-agnostic.)

Here's how the "hugely laborious manual process" went, according to Bell and Neville:

Get a hold of every Xbox 360 guitar and drum controller they wanted to support in Rock Band 4. Wirelessly connect one to the adapter. Try everything on the instrument — press all the buttons, hit all the pads, flick all the switches, push the whammy bar or pedal, tilt the unit, and more — to capture all the input data. Repeat for all the other controllers.

Part of the process involved tracking down an Xbox 360 engineer who no longer works for Microsoft to get some additional development insight. That was "part of Microsoft's assistance on the project," according to Bell and Neville.

"It had to be seamless for the gamer but I assure you a great deal of programming work went on in the background to make this possible," they continued. "There were points in development where we were truly unsure if the adapter would be viable and even work."

The product

The undertaking resulted in a device that Harmonix and Mad Catz call the Legacy Game Controller Adapter, a name that is descriptive if nothing else. The USB dongle is included in the retail Xbox One version of Rock Band 4, and raises the price of the game from $59.99 to $79.99.

Mad Catz also sells the adapter separately for $24.99, although it was hard to find in stock when the game launched last month. Neville and Bell told Polygon that the dongle "[contains] key components which are in limited supply," a bottleneck that affected Mad Catz's ability to produce the unit in large quantities, but said the company is making more as fast as it can. (The adapter is now back in stock at Amazon, the only retailer where it is available.)

A price hike of $20 is a not-insignificant bump, and could have felt like a bitter pill for Xbox gamers to swallow. Harmonix and Mad Catz had to walk a fine line in explaining why the adapter was necessary, pointing out the differences in wireless connectivity between the Xbox 360 and Xbox One without saying something as blunt as "don't blame us; blame Microsoft."

The USB dongle was the only logical option on Xbox, and buying it is still much cheaper than paying for new instruments — the full-band bundle for Rock Band 4, which includes the game, one guitar, a drum kit and a microphone, costs $249.99. Bell and Neville also noted that at the price Mad Catz set for the adapter, neither Mad Catz nor Harmonix is making a profit off the device: "What it costs us to manufacture and ship into distribution is what the consumer is paying," they said.

"In this particular case, it's the right thing to do," said Sussman.