Christopher L. Winfrey
Chief Financial Officer at Charter Communications
Thanks, Tom. As we discussed last quarter, given the effects of COVID in 2020, 2019 remains the better customer growth comparison for 2021. We'll continue to reference the COVID schedules we've provided last year and included again on slides 17 and 18 of today's presentation to help with the year-over-year financial comparisons. Turning to our results on slide five. We grew total residential and SMB customer relationships by 1.3 million in the last 12 months and by over 330,000 in the second quarter. Including residential and SMB, we grew our Internet customers by 400,000 in the quarter and by 1.5 million or 5.5% over the last 12 months. Video declined by 50,000 in the second quarter, and wireline voice declined by 78,000. In residential Internet, we added a total of 365,000 customers in the quarter, higher than the 221,000 that we gained during the second quarter of 2019.
Our residential video customers declined by 63,000 less than the loss of 150,000 we saw in the second quarter of 2019. In wireline voice, we lost 99,000 residential customers in the quarter, also less than the loss of 207,000 in the second quarter of 2019, and that was driven by continued fixed to mobile substitution. Turning to mobile. We added 265,000 mobile lines in the quarter. And as of the end of the quarter, we had 2.9 million mobile lines. Despite the lower number of selling opportunities from cable sales, we continue to drive mobile growth with our high-quality, attractively priced service rather than using device subsidies. A few things to keep in mind when reviewing this quarter's customer results. First, we estimate that 60,000 of our residential Internet net adds would not have occurred without the Emergency Broadband Benefit program, or EBB, which launched in May. These incremental Internet net adds had little impact on our video and voice net adds.
Some of what we estimate as business-as-usual sales also enrolled in the EBB program, as did some of our existing customers. Those customers did not impact our second quarter customer net adds. Second quarter customer net adds also benefited from certain state-mandated moratoriums on Internet, video and voice disconnects. Internet benefited by about 40,000, with video and voice net additions also benefiting, but by loss. Some states have recently ended their moratoriums. So similar to our KAC customers last year, we will work with these customers to forgive portions of their bills and provide financing options to customers. And we expect to keep them as customers, same as we did with the KAC program. Looking at the bigger picture. Residential customer activity levels in the marketplace, including sales, churn and particularly nonpay churn, are taking a bit longer than we expected to return to normal levels.
As a result, our first half 2021 financials have been better than we expected, driven by lower operating expense given lower transactions and significantly lower bad debt. We continue to expect transaction volume to pick up in the second half of this year, driving more selling opportunities in the market for cable and mobile, and we still expect full year Internet and customer relationships to be at or above 2019 net additions. So the financial effects that we expected of a higher churn environment and expected higher sales for Charter as a share taker could accrue later in 2021 or even into 2022. Moving to financial results, starting on slide six.
Over the last year, we grew total residential customers by 1.2 million or 4.1%. Residential revenue per customer relationship increased by 1.8% year-over-year given last year's second quarter residential revenue write-down of $76 million for customers in the Keep Americans Connected program as well as bill credits that we provided last year as part of the remote education offer, which provided two months of free Internet. Those onetime comparison benefits were partly offset by the same bundle and mixed trends we've seen over the past year, including a higher mix of nonvideo customers and a higher mix of choice, essentials and stream customers within our video base. Keep in mind that our residential ARPU does not reflect any mobile revenue. slide six shows residential revenue grew by 6.8% year-over-year, reflecting customer relationship growth and last year's COVID impacts.
Turning to commercial. SMB revenue grew by 6%, and this growth rate reflects COVID-related impacts of $17 million that negatively impacted the second quarter of 2020. Excluding this impact from last year, SMB revenue grew by 4.2%, faster than last quarter's growth. Enterprise revenue was up by 5.1% year-over-year and was also negatively impacted last year by $18 million due to COVID credits. Excluding this impact from last year, enterprise revenue grew by 2% and by 5.8% when additionally excluding wholesale revenue. Enterprise PSUs grew by 3.7% year-over-year. First quarter advertising revenue increased by 65% year-over-year, primarily due to COVID impact last year. When compared to the second quarter of 2019, advertising revenue grew by 4%, primarily due to our growing advertising -- advanced advertising capabilities, partly offset by lower local ad revenue.
Mobile revenue totaled $519 million, with $214 million of that revenue being device revenue. In total, second quarter revenue was up 9.5% year-over-year. Moving to operating expenses on slide seven. In the second quarter, total operating expenses grew by $575 million or 8% year-over-year. Similar to revenue, the year-over-year operating expense growth rate is elevated due to 2020 COVID effects. Programming increased 3.6% year-over-year due to higher rates, offset by a higher mix of lighter video packages, such as Choice, Essentials and Stream. Regulatory, connectivity and produced content grew by 36.9%, driven by more Lakers games than normal this quarter given the delayed start to the NBA season combined with no Lakers or Dodger games expensed in the prior year due to COVID-19. Excluding sports rights costs related to our RSNs, this expense line item grew by 3.2% year-over-year.
Cost to service customers declined by 1.2% year-over-year compared to 4.2% customer relationship growth. The decline was driven by lower transaction costs and lower bad debt, partly driven by government stimulus packages. Excluding bad debt, cost to service customers was flat year-over-year despite a higher number of customers and outsized hourly wage increases that we put through earlier this year. Marketing expenses grew by 3.1% year-over-year, driven by second quarter 2020 COVID impacts, including lower media placement rates in 2020 and a payroll tax credit. Mobile expenses totaled $586 million and were comprised of mobile device cost tied to device revenue, customer acquisition and service and operating cost.
And other expenses grew by 13.5%, driven primarily by higher corporate costs and advertising sales expense given the strength of ad sales this quarter, combined with the weakness in the ad market in the prior year. Adjusted EBITDA grew by 11.8% in the quarter. And turning to net income on slide eight, we generated $1 billion of net income attributable to Charter shareholders in the second quarter versus $766 million last year. The year-over-year increase was driven by higher adjusted EBITDA. Turning to slide nine. Capital expenditures totaled $1.9 billion in the second quarter, in line with last year's second quarter capital spend, driven by higher scalable infrastructure spend, primarily related to augmentation of our network capacity at our normal pace for customer growth and usage with incremental spending to reclaim the network headroom we maintained prior to COVID.
This was offset by lower spend on modems, routers and self-installation kits given the elevated sales volume in the second quarter of last year. We spent $124 million on mobile-related capex this quarter, which is mostly accounted for in support capital and was driven by investments in back-office systems and mobile store build-outs. For the full year 2021, we continue to expect cable capital expenditures, excluding the RDOF investments, to be relatively consistent as a percentage of cable revenue versus 2020. As slide 10 shows we generated nearly $2.1 billion of consolidated free cash flow this quarter, an increase of 10.8% year-over-year. We've finished the quarter with $87.5 million in debt principal.
Our current run rate annualized cash interest, pro forma for financing activity completed in July, is $4 billion, $4.0 billion to be exact. As of the end of the second quarter, our net debt to last 12-month adjusted EBITDA was 4.3 times. We intend to stay at or just below the high end of our four to 4.5 times leverage range. In June, we converted Advance/Newhouse's preferred partnership units, which had a face value of $2.5 billion and paid a 6% coupon. They were converted into 9.3 million common partnership units, which means we no longer pay $150 million in preferred dividends per year. During the quarter, we repurchased 6.1 million Charter shares and Charter Holdings common units, totaling about $4 billion at an average price of $656 per share.
Since September of 2016, we've repurchased $47 billion or 36% of Charter's equity at an average price of $421 per share. So we have a successful operating model and growth-oriented investment approach, which when coupled with the unique balance sheet structure and improving capital allocation strategy, has and will produce cash flow growth and shareholder value for years to come. Operator, we're now ready for Q&A.