AMP Bank changed its preferred funding mix during the June half, supporting the growth in its mortgage book with wholesale debt and allowing its deposit book to run down.The bank provided details of the change to its funding strategy in its interim results' presentation, which was released yesterday.The bank's mortgage book stood at A$12.8 billion in June - three per cent higher than at the same time last year.The deposit book fell by two per cent, to $8.1 billion. The bank said the reduction was due an adjustment to deposit rates, following the change to its preferred funding mix.Funding for the growth in mortgages came from two issues of medium-term notes, worth a total of $800 million. The $800 million was made up of a three-year, $500 million senior unsecured floating-rate note, issued at 108 basis points over the bank bill swap rate, and a four-year, $300 million senior unsecured floating-rate note, issued at 110 bps over swap. AMP Bank contributed $38 million to AMP's group operating earnings during the six months to June - up from $29 million in the previous corresponding period.The bank's return on capital was 15.4 per cent - up from 13.4 per cent in the previous corresponding period.Costs rose eight per cent, largely due to higher commissions and other mortgage acquisition costs. However, the cost-to-income ratio fell from 36 per cent to 32 per cent.Revenue was $105 million for the six months to June - up 18 per cent, from $89 million for the previous corresponding period. The net interest margin rose from 1.22 per cent, in the June half last year, to 1.39 per cent in the latest half.Among business developments during the half, the bank worked with AMP's expanded self-managed superannuation fund division to develop a new loan product, AMP SuperEdge.The bank also launched an innovative deposit product, AMP Notice Account. The account offers savers a variable rate that is at least 100 basis points above the cash rate in return for a requirement to give 31 days' notice before withdrawing funds.